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Lithiated silicon (Li15Si4) has a theoretical specific capacity of 3579 mAh g-1 compared to 372 mAh g-1 for graphite at conventional conditions. However, the large volume fluctuations during (de)lithiation result in irreversible damage to the solid-electrolyte interphase (SEI) and then gradual capacity loss as the electrolyte is reduced to reform the surface passivation layer in each cycle. One approach to mitigate this issue is to develop an artificial SEI layer on the silicon surface to limit strain-induced fracture during cycling. The approach presented herein is unique in that both surface functionalization and homogenous doping of the silicon are simultaneously achieved by introducing a trace amount of phosphorus via facile solid-state synthesis. Increased thermodynamic stability of the Li-P-Si ternaries as evidenced in previous studies has been corroborated herein, showing that the P-doped lithium silicide exhibits a contracted crystal lattice which both reduces the Li+ mobility and suppresses the chemical reaction between lithium silicide and typical organic electrolytes. The effect of P-doping, specifically the doping amount and depth within the silicon particles, on the electrochemical performance in lithium-ion batteries was investigated. In conclusion, the solid-state reaction between red phosphorus and nanoparticulate silicon is a relatively simple and tunable method enabling versatile studies of the interfacial chemistry of silicon electrodes over hundreds of cycles.

Olivine structured lithium cobalt phosphate, LiCoPO4 (LCP), is an attractive cathode material that has a theoretical energy density of 802 Wh/kg based on LCP’s 4.8 V redox potential and 167 mAh/g theoretical capacity. The investigation of this material was first reported in 2000 by Amine et al. [1] realizing a capacity of only 70 mAh/g. The low capacity was attributed to both the poor electronic conductivity of LCP and the initiation of the electrolyte salt decomposition [1]. Considerable efforts worldwide since then made on improving the utilization of LCP through the reduction of particle size, carbon blending, carbon coating and morphology improvement have led to an increase of capacity utilization to 140 mAh/g [2]. Efforts have also been made in developing compatible high voltage electrolytes [3]. However, these efforts had not resulted in a rechargeable LCP without substantial loss of capacity in limited cycles. A breakthrough in improving the cycle life of LCP against Li up to 500 times with 80% capacity retention was made by substituting part of Co by Fe, which was first reported by Allen et al. in 2011 [4]. Further substitutions by Cr and Si in Fe substituted LCP (s-LCP) with improved electrolyte improve the discharge capacity to 140 mAh, coulombic efficiency to 99.7% and cycle up to 250 times with almost no capacity fade [5]. The success in realizing the potential of this material is attributed to the success in modifying the electronic structure and stabilizing the electrochemical stability of LCP through atomic level substitutions. Good cycle life has also been demonstrated in full cells made of graphite anode and s-LCP cathode in coin cells and 1.2 Ah pouch cells [6]. This talk will present a progression of the s-LCP based Li-ion cells development and the importance of the atomic level engineering in improving the performance of the high voltage cathodes. References K. Amine, H. Yasuda, M. Yamachi, Olivine LiCoPO4 as 4.8 V Electrode Material for Lithium Batteries, Electrochem. Solid-State Lett. 2000, 3(4) 178-179. S.-M. Oh, S.-T. Myung, Y.-K. Sun, Olivine LiCoPO4–carbon composite showing high rechargeable capacity, J. Mater. Chem., 2012, 22, 14932. R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M. A. Schmidt, N. Schall, C. Bruenig, Electrolyte solution for the improved cycling performance of LiCoPO4/C composite cathodes. Electrochem. Commun. 2013, 28, 20-23. J. L. Allen, T. R. Jow, J. Wolfenstine, Improved cycle life of Fe-substituted LiCoPO4, J. Power Sources, 2011, 196(20), 8656-8661. J. L. Allen, J. L. Allen, T. Thompson, S. A. Delp, J. Wolfenstine, T. R. Jow, Cr and Si Substituted-LiCo0.9Fe0.1PO4: Structure, Full and Half Li-ion Cell Performance, J. Power Sources, 2016, 327, 229-234. D. Liu, W. Zhu, C. Kim, M. Cho, A. Guerfi, S. A. Delp, J. L. Allen, T. R. Jow and K. Zaghib, High-Energy Lithium-Ion Battery Using Substituted LiCoPO4: From Coin Type to 1Ah Cell, J. Power Sources, 2018, 388, 52-56.

There is a constant drive to improve the specific energy and energy density of the state-of-the-art lithium-ion battery. High energy is achieved by either choosing a cathode material that operates at a higher potential or has a higher specific capacity. Compared with conventional 4-V cathodes such as LiCoO2 (LCO), LiNi x Co y Al z O2 (NCA), LiMn2O4 (LMO) and 3.5-V LiFePO4 (LFP), LiCoPO4 (LCP) operates at 4.8 V (vs. Li/Li+) with a theoretic capacity of 167 mAh g-1. The higher voltage of LCP results in specific energy of ~800 Wh kg-1, which is about 25% higher than that of conventional cathodes in lithium-ion batteries. Although Co is more expensive than the other transition metals, the energy cost of LCP is expected to be less than other commercialized lithium-ion batteries on the market [1] due to the improved energy density. LCP suffers from severe capacity fade due to the low intrinsic electronic/ionic conductivity, structural deterioration and electrolyte decomposition [2, 3]. A diverse range of synthesis strategies, such as planetary milling, microwave heating and spray-pyrolysis et al, were explored to yield smaller particles and/or composites, but the above-mentioned shortcomings and the electrochemical performance remain unsatisfied [4-6]. Deposition of carbon coatings or precipitation of Co2P under high-temperature annealing in inert atmosphere produced a significant increase of over 105 in the electrical conductivity of LCP [7]. In addition, electrolyte additives were also employed to improve the attractiveness of LCP [8, 9]. These two approaches, however, do little to stabilize the cathode material itself, or only protect the electrolyte from decomposition during cycling. In this work, substitution of Cr, Fe and Si, as well as the use of a carbon-coating, improved the performance of LiCoPO4. The structural analyses and electrochemical properties are discussed. Cr, Fe and Si were added to improve the performance of olivine LiCoPO4 in cathodes for lithium-ion batteries. A substituted-LiCoPO4 in a half cell delivered a reversible capacity of 125 mAh/g at C/3 rate, with no capacity loss after over 100 cycles at 25 °C. The well-known capacity fade of LiCoPO4-based cathodes was almost completely eliminated by substituting Cr, Fe and Si. The electrochemical data of coin type battery and 1 Ah laminate cell will be shown. Acknowledgments Financial support from Hydro-Quebec and the US Army Research Lab is gratefully acknowledged.

High voltage olivine structured cathodes offer the potential for high energy dense storage. LiCoPO4 (LCP) has a theoretical discharge capacity of 167 mAhg-1 at 4.8 V. Its adoption has been limited by poor cycling owing to both structural instability of the de-lithiated material and electrolyte instability at 5 V. LiNiPO4 (LNP) with a similar discharge capacity has a discharge voltage of 5.1 V for an even greater energy storage potential. We have shown that ion substitution is a powerful method to stabilize the cycling of the LCP high voltage electrode material in its charged, fully de-lithiated state. A low level of Fe substitution for Co increases the electrical and ionic conductivity and stabilizes the structure through change in the electronic structure which shift the redox activity toward Fe/Co and away from oxygen. Substitution of Cr for Co further increases the redox activity of Co leading to higher discharge capacity than for Fe-only substituted LCP. This stabilization mechanism is confirmed through spectroscopic measurements, through DFT calculation and is evidenced by extremely reduced discharge capacity fade during electrochemical cycling. Addition of Si to the cathode reduces the reactivity with the electrolyte as evidenced by an increase in the coulombic efficiency. LNP is even more challenging with extremely low electrical conductivity and an unfavorable electronic structure. We will discuss attempts to improve its performance. Electrolyte stability has been addressed through sacrificial additives, changes in solvent composition and surface modification of LCP. This paper will touch upon the most recent results in the development of a stabilized high voltage olivines with high energy and excellent cycle life and the development of supporting electrolytes.

Current state of the art lithium ion batteries have reached a plateau in energy density. Next generation materials are becoming available and researched. There are two directions being pursued to achieve higher energy density. The first path is to use materials that have higher energy density than the ones currently used, for example, incorporating silicon into the anode or transitioning to lithium as the anode. A second direction is to use cathode materials that are electrochemically active at higher voltages. Lithium cobalt phosphate (LCP) is one example of a cathode material that operates at a higher voltage than state of the art materials (4.8 V vs Li). Pure LCP suffers from severe capacity fade due to structural degradation during cycling. Recently we have been able to increase the cycle life of LCP by the introduction of elemental substitution of iron, chromium and silicon. The major challenge now is that conventional electrolytes are not stable at the elevated voltages at which LCP cycles. The electrolytes must not only be stable at high voltages but also at low voltage if graphite is used as the anode material. Generally, when the oxidative stability voltage of the electrolyte increases, the reductive stability voltage increases causing problems on the anode. The progress of electrolyte development for LCP batteries will be discussed.

Lithium (Li)-ion battery (LIB) and electric double-layer capacitor (EDLC) are two widely used electrochemical energy storage devices. LIB is made with Li intercalated graphite anode and Li metal oxide cathode and EDLC is made with high surface activated carbon for both anode and cathode. To compare two energy storage devices, LIB has a higher specific energy (or energy density) than EDLC, but has low specific power (or power density) and poor cycle life than EDLC as shown in Fig. 1. In high energy and low power applications, the LIB is the best choice; in high power and long cycle applications, the EDLC is used; however, in many applications, the power requirement is not a constant but rather spans over a range of power levels. It has been demonstrated that when batteries and supercapacitors were externally connected to form a hybrid power source, the voltage drop during the pulsed current for the hybrid power source was less than that for the battery, and operating time of the hybrid power source was significantly extended [1,2]. However, because the maximum cell voltage of single cell EDLC is less than that of LIB; therefore, hybrid power sources could only be made with two separate battery and EDLC cells and electrically connect them with or without electric circuits. The Li-ion capacitor (LIC) is a new energy storage device which consists of an EDLC cathode and a LIB anode, between which the ions shuttle during charge and discharge processes. The LIC not only retained all the advantages of EDLC such as specific power >5 kW/kg and cycle life > 100,000 cycles; but also had higher specific energy of 15-30 Wh/kg and higher maximum cell voltage of 4.1 V than that of EDLC [3,4]. Because the potentials of anode and cathode as well as the maximum cell voltage of LIC is comparable to that of LIB, it allows the LIC and LIB to be assembled in one package as a monolithic LIB and LIC hybrid cell. Very recently, we have demonstrated a new hybrid energy storage device that combines the advantages of both the LIB and the LIC, thereby avoiding their inherent defects, while bridging the gap between the high energy densities offered by batteries and the high power densities seen in LIC as shown in Fig. 1. The energy density and power density of the hybrid cell can be designed to meet the requirements by a reasonable distribution of the ratio between LIB and LIC electrode materials in the internal hybrid cell. For example, in a LIB/LIC internal hybrid cell, made with a 20 wt.% LiFePO4 (LFP) and 80 wt.% activated carbon (AC) mixed cathode and a pre-lithiated hard carbon anode, the specific energy increased by 40% to compare with LIC. As can be seen in Fig. 2(a), the discharge profile can be divided into two parts: at cell voltage range of 2.2-3.0 V and 3.3-3.8 V, the capacity was contributed by capacity material, while at 3.0-3.3 V, it was contributed by battery material. It can also be seen from Fig. 2, that the capacity contribution by capacitance material was less sensitive than battery material when the discharge rate increased from 0.1C to 53C. Fig. 2(b) shows the charge-discharge voltage profiles measured at an interruption rate of 0.2C as a function of cycle number after the hybrid cell was cycled at 43C rate. It must be notified that cycle life of the hybrid energy storage cell was found to be prolonged significantly as shown in Fig. 2(c). After 20,000 cycles, the degradation of specific capacity for both capacitance and battery materials inside hybrid cell was less than 5%. As a comparison, the specific capacity of a battery cell using LFP cathode, as a control experiment, decreased by 40% after 2,000 cycles. Reference: [1] J.P. Zheng, S.P. Ding, and T.R. Jow, “Hybrid Power Sources for Pulse Current Applications”, IEEE Transactions on Aerospace and Electronic Systems, 37, 288 (2001). [2] D. Shin, Y. Kim, Y. Wang, N Chang, and M. Pedram, “Constant-current regulator-based battery-supercapacitor hybrid architecture for high-rate pulsed load applications”, J. Power Sources, 205, 516 (2012). [3] W.J. Cao and J.P. Zheng, “Li-ion Capacitors with Carbon Cathode and Hard Carbon/SLMP Anode Electrodes”, J. Power Sources, 213, 180 (2012). [4] W.J. Cao, J. Shih, J.P. Zheng, and T. Doung, “Development and characterization of Li-ion capacitor pouch cells”, J. Power Sources, 257, 388 (2014). Figure 1

To improve the power performance of Li-ion batteries, it is important to understand the factors that limit the Li+ charge transfer kinetics. Li-ion batteries comprised of graphite anode and lithium cobalt oxide cathode in an electrolyte made of 1 M LiPF6 in EC-DMC-DEC carbonate solvent mixtures could not deliver their room temperature capacity at a rate of C/2 at -30 and -40 oC [1]. When DEC was replaced by linear ester solvent, such as ethyl acetate (EA) or methyl butyrate (MB), the Li-ion batteries at -30 and -40 oC could deliver over 80% of their room temperature capacity at the same rate [2]. When the LiPF6 salt is replaced by LiBOB in EC-EMC (1:1 wt. ratio) carbonate solvent mixture, the impedance of graphite-electrolyte interface measured using graphite/Li half cells in the electrolyte with LiBOB is 3 times that in the electrolyte with LiPF6 [3]. These examples show that the electrolyte components play crucial roles in affecting Li+ charge transfer kinetics in Li-ion batteries. The Li+ charge transfer process starts from the solvated Li+ in the electrolyte to the reception of an electron (e-) from the electrode. This involves the de-solvation step of Li+ before entering into SEI and the diffusion step of Li+ through the SEI at the electrode and electrolyte interfaces before receiving an e- from the electrode at the electrode and SEI interface. Abe et al. [4-6] believe that the Li+ desolvation is the rate limiting step, which is supported by the results from the investigation of the electrolytes of different solvent systems using LiClO4 salt at HOPG, Li4Ti5O12 and Li+ solid conductors interfaces. The question is whether the de-solvation as a limiting step can be extended to the cathode-electrolyte interface. Jow et al. [7] examined the Li+ charge transfer kinetics at the graphite anode-electrolyte interface (SEI) and LiFePO4 (LFP) cathode-electrolyte interface (CEI) in a full cell, LFP/Gr, with Li as a reference electrode in 1 M LiPF6 in EC-DMC-MB with VC as an additive. It is found that the activation energy (Ea) at the graphite-electrolyte is about 67 kJ mol-1, which is much higher than 33 kJ mol-1 found at the LFP-electrolyte interface. It is concluded that the electrodes and their associated electrode-electrolyte interfacial layers are controlling the Li+ charge transfer kinetics. The impact of additives such as VC, LiBOB, and LiFSI, etc. on the impedance of the anode-electrolyte and the cathode-electrolyte interfaces in LiNiCoAlO2/Gr cylindrical cells in 1.0 M LiPF6 in EC-EMC-MP (20:20:60 vol %), where MP is methyl propionate, at low temperatures was studied by Jones et al. [8]. It is found that the additive impacts the impedance at the anode differently from that at the cathode. Further, we also found that the additives impact the Ea of Li+ charge transfer differently at the anode-electrolyte and the cathode-electrolyte interfaces. Different electrolyte components, including additives, result in different reduction and oxidation reactions during cell formation and cycling at the anode and cathode [9] and therefore, different and SEI and CEI layers. A discussion regarding the conditions in which the de-solvation step or the Li+ diffusion in either SEI or CEI layer is limiting the kinetics will also be provided. Acknowledgement Some work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). References M. C. Smart, B. V. Ratnakumar, and S. Surampudi, J. Electrochem. Soc., 1999, 146(2), 486-492. S. Herreyre, O. Huchet, S. Barusseau, F. Peron, J. M. Bodet, Ph. Biensan, J. Power Sources, 2001, 97-98, 576. K. Xu, J. Electrochem. Soc., 2008, 155(10), A733-A738. T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc., 2004, 151(8), A1120-A1123. Y. Ishihara, K. Miyazaki, T. Fukutsuka, T. Abe, ECS Electrochemistry Lett., 3 (8) A83-A86 (2014). T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama, and Z. Ogumi, J. Electrochem. Soc., 2005, 152(11), A2151-A2154. T. R. Jow, M. B. Marx, J. L. Allen, J. Electrochem. Soc., 2012, 159 (5), A604-A612. J.-P. Jones, M. C. Smart, F. C. Krause, B. V. Ratnakumar, E. J. Brandon, ECS Trans., 2017, 75, 1-11. S. A. Delp, O. Borodin, M. Olguin, C. G. Eisner, J. L. Allen, T. R. Jow, Electrochimica Acta, 2016, 209, 498-510.

There is a constant drive to improve the specific energy and energy density of the state-of-the-art lithium-ion battery. High energy can be achieved by either choosing a cathode material that operates at a higher potential or has a higher specific capacity. Compared with the conventional 4-V cathodes such as LiCoO2 (LCO), LiNi x Co y Al z O2 (NCA), LiMn2O4 (LMO) and 3.5-V LiFePO4 (LFP), LiCoPO4 (LCP) operates at 4.8 V (vs. Li/Li+) with a theoretic capacity of 167 mAh g-1, resulting in a specific energy of ~800 Wh kg-1, which is about 25% higher than those of the conventional LFP and LMO lithium-ion batteries. Although Co is more expensive than the other transition metals, the energy cost of LCP is expected to be cheaper than all commercialized lithium-ion batteries on the market [1] due to the improved energy density (FIG.1). However, LCP suffers from severe capacity fade due to the low intrinsic electronic/ionic conductivity, structure deterioration and electrolyte decomposition [2]. In order to improve the cyclability and reduce the cost of LCP, Co ions were partially substituted by cheaper elements such as Fe, Cr and Si etc [3]. Meanwhile, a carbon-coating was used to improve the electronic conductivity of LCP. Electrochemical tests showed that both carbon-coating and substitution greatly improved the cycling performance of LCP, which suggests that the substituted LCP is a very promising cathode candidate for high energy lithium-ion battery. References [1] S. Brutti, S. Panero, ACS Symposium Series, 1140, Chapter 4, 69 (2013). [2] K. Tadanaga, F. Mizuno, A. Hayashi, T. Minami, M. Tatsumisago, Electrochemistry 71, 1192 (2003). [3] J. L. Allen, J. L. Allen, T. Thompson, S. A. Delp, J. Wolfenstine, T. Richard Jow, J. Power Sources, 327, 229 (2016). FIG. 1. Energy density and energy cost of LCP compared with other cathode materials currently used in lithium-ion batteries. Figure 1

The need for environmentally friendly, safe, stable, and low cost materials for application in lithium-ion batteries has led to strong interest in developing olivine-type LiMPO4 (M = Fe, Mn, Co, Ni) cathode materials. Among them, LiFePO4 (LFP) has been extensively studied and commercialized. However, LFP has some special shortcomings in practical applications, such as low potential plateau (3.4 V vs. Li/Li+) and small packing density (due to the inclusion of large-volume carbon), which lead to relatively low specific energy density (580 Wh kg-1). Whereas LiCoPO4 (LCP) in the olivine family has been considered as an attractive cathode candidate due to the large theoretical capacity (167 mAh g-1), high operating voltage (4.8 V vs. Li/Li+) and high specific energy density (800 Wh kg-1). However, LCP suffers from severe capacity fade due to the low intrinsic electronic/ionic conductivity, structure deterioration and electrolyte decomposition [1]. In this work, Hydro Quebec and US Army Research Laboratory dedicated to develop the high voltage olivine cathode. Specifically, partially Co-substitution strategy with a carbon coating was successfully used to improve the cycling stability of LCP cathode [2]. FIG. 1 shows the cyclability of substituted-LCP olivine cathode at room temperature under C/3 rate, from which no obvious capacity loss was observed in 100 cycles. Moreover, phosphate based cathodes may provide higher abuse tolerance than oxides at a given voltage due the strong covalent band of P-O. Therefore, the substituted high voltage LCP olivine cathode has the apparent potential to be a next decade success story in lithium-ion technologies and to find large application in the next generation lithium-ion batteries. References [1] K. Tadanaga, F. Mizuno, A. Hayashi, T. Minami, M. Tatsumisago, Electrochemistry 71, 1192 (2003). [2] J. L. Allen, J. L. Allen, T. Thompson, S. A. Delp, J. Wolfenstine, T. Richard Jow, J. Power Sources, 327, 229 (2016). Figure 1

Lithium-ion capacitors (LICs) were investigated for high power, moderate energy density applications for operation in extreme environments with prolonged cycle life performance. The LICs were prepared as three-layer pouch cells with an asymmetric configuration employing Faradaic pre-lithiated hard carbon anodes and non-Faradaic anion adsorption−desorption activated carbon (AC) cathodes. The studies were conducted in order to develop and improve the LIC cell design, the cell formation procedure, the stability of the electrolytes and additives, and the long-term capacity retention under high stress conditions. The LIC cells have been evaluated using critical performance tests under the following conditions: long-term cycling stability at room temperature (2.2-3.8 V), high temperature holds at 3.8 V, and high upper cut-off potential conditions (2.2 V to 3.8-5.0 V). The rate performance of different electrolytes and additives were measured after the LIC cell formation by charging at a 1C rate and discharging at a 1C, 2C, 4C, 6C, 10C, and again 1C rate from 2.2 V to 3.8 V. The performance of the energy storage devices were evaluated in terms of the specific discharge capacity, operational voltage/temperature window, coulombic efficiency, and cycling stability under different conditions using various electrolytes and additives. The presence of electrolyte additives were found to be essential to the improved performance of the LIC cells at room temperature, at high temperature, and at higher cut-off potentials.

The rate performance of Li-ion batteries remains challenging, especially at temperatures below -20 oC. Apart from engineering improvements, the rate performance of the cell is determined by the rate of Li + ion transfer across the interface, so-called charge transfer kinetics and the rate of Li+ ion diffusion within the electrode. In this paper, we will focus on the Li + ion charge transfer kinetics as it is still poorly understood and can be improved by modifying the electrolyte. Examination of Li + charge transfer kinetics at both a graphite anode and a LiFePO4 cathode, revealed a much higher activation at the graphite anode/electrolyte interface than at the LiFePO4 cathode/electrolyte interface [1]. Values from 50 to 60 kJ mol-1 have been observed for the activation energy at the graphite/electrolyte interface [1-3] whereas, at the LiFePO4 cathode/electrolyte interface, an activation energy of 30 kJ mol-1 has been observed [1]. At either electrode, the solvated Li + ion needs to be de-solvated before diffusing into the electrode. The distinct difference between the graphite anode and the intercalating cathode is that there is a distinct presence of a solid electrolyte interphase (SEI) on the graphite anode while there is no SEI or no discretely defined SEI layer on the cathode. What is not clear is why the SEI on the graphite anode leads to a larger Li + charge transfer activation energy value. Is this due to diffusion through the SEI itself or the slow de-solvation process in the presence of the SEI? Recent studies indicate that the electrolyte composition plays a vital role in determining the electrolyte solvation structure and furthermore the nature of the SEI formed on the graphite anode can be modified by changing solvents, salt concentrations and additives [3-5]. This paper will report our recent exploration of how changing the electrolyte composition can improve the Li + charge transfer kinetics. References T. R. Jow, M. B. Marx, J. L. Allen, J. Electrochem. Soc. 2012, 159 (5), A604-A612. T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 2004, 151(8), A1120-A1123. Y. Yamada, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir, 2009, 25(21), 12766-12770. M. Nie, D. P. Abraham, D. M. Seo, Y. Chen, A. Bose, and B. L. Lucht, J. Phys. Chem. C 2013, 117, 25381−25389. K. Abe, M. Colera, K. Shimamoto, M. Kondo, and K. Miyoshia, J. Electrochem. Soc. 2014, 161 (6) A863-A870.

There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines based on LiCoPO4 are promising cathode materials to help fulfill this need owing to their relatively high discharge capacity of up to 167 mAh g-1 at a discharge potential of 4.8V [1]. However, LiCoPO4 based Li-ion cells show a severe loss of discharge capacity upon multiple charge – discharge cycles owing to structure deterioration and electrolyte decomposition [2]. Partial substitution of Co by Fe significantly improves the cycle life [3] and increases Li+ ion and electrical transport [4]. Our most recent studies show that multi-element substitution for Co in LiCoPO4 further reduces capacity fade, substantially improves discharge capacity and reduces reactivity of the cathode with the electrolyte. Currently, we have achieved an energy storage capacity of 670 Wh kg-1 of cathode material (84% of LiCoPO4 theoretical) which can be compared to a maximum theoretical 576 Wh kg-1 for commercialized LiFePO4. The capacity fade of Li / LiCo1-xMxPO4 half cells over 500 cycles is less than 6%. This paper will report on our most recent results. We will discuss synthesis methods, transport measurements, electronic structure[5], the most promising substitutional chemistry and the structure of substituted LiCoPO4 as well as performance enhancements that result from improving the electrolyte high voltage stability [6]. References K. Amine, H. Yasuda, M. Yamachi, “Olivine LiCoPO4 as 4.8 V electrode material for lithium batteries,” Electrochem. and Solid State Letters, 3 (2000) 178. J. Wolfenstine, U. Lee, B. Poese, J.L. Allen, “Effect of oxygen partial pressure on the discharge capacity of LiCoPO4,” J. Power Sources 144 (2005) 226. J.L. Allen, T.R. Jow, J. Wolfenstine, “Improved cycle life of Fe-substituted LiCoPO4,” J. Power Sources 196 (2011) 8656. J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow, J. Wolfenstine, “Transport Properties of LiCoPO4 and Fe-substituted LiCoPO4,” J. Power Sources 254 (2014) 204. M.D. Johannes, K. Hoang, J.L. Allen, K. Gaskell, “Hole polaron formation and migration in olivine phosphate materials,” Phys. Rev. B 85 (2012) 115106. J.L. Allen, J.L. Allen, S.A. Delp, T.R. Jow, “Electrolyte additives for reducing the irreversible capacity loss, impedance and polarization of a doped LiCoPO4 cathode,” ECS Transactions 66 (2015) 149. Figure 1

Extensive research on next generation lithium ion batteries, specifically batteries operating at high voltage (>4.4V), is currently underway. There are several issues that need to be addressed in order to produce a viable product. For example, graphite anodes are used in current state of the art batteries but silicon-based anodes offer higher energy density. Also, cathode materials that are electrochemically active at high voltages are available, e.g. LiNi0.5Mn1.5O4 (LNMO) @ 4.7V and LiCoPO4 (LCP) @ 4.8V. Unfortunately, batteries composed of these materials with the state of the art electrolyte suffer from poor cycle life. New electrolytes are essential for the operation of next generation high voltage lithium ion batteries. This research focused on the electrolyte for LNMO-graphite based batteries. The importance of not only the high voltage stability of the electrolyte but also the low voltage stability will be discussed. Cyclic voltammetry (CV) on glassy carbon electrodes allow for the investigation of both the oxidative and reductive stability of the electrolytes. Cycle life data, Coulombic efficiency, and fade rate data was collected from LNMO-graphite coin cells. Differential capacity (dQ/dV) vs. V plots were also constructed from the coin cell data to show how the electrochemical behavior changed over time. The dQ/dV vs V data for the state of the art electrolyte show a buildup of impedance over time which was also evidenced via electrochemical impedance spectroscopy (EIS). Several different lithium salts and additives were investigated in this study. The results suggest that while the oxidative stability of the electrolyte is important, the reductive stability must not be ignored.

The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both a function of ion substitution and oxidation state. Soft Ex situX-Ray absorption spectroscopy (XAS) made it possible to compare the changes in chemical bonding between electrode bulk and surface as a function of lithium content. This technique can probe the density of states at the transition metal and O levels. The evolution of these levels revealed changes in the metal-oxide covalence when lithium was deintercalated from the structure, and, thus, the material was oxidized. An increase in covalence can lead to the destabilization of the anions. If this process takes place in the bulk of the material, this destabilization can lead to thermal degradation via oxygen loss. At the surface, even small degrees of destabilization are sufficient to produce oxidizing species that attack the electron-rich solvent molecules in the electrolyte, leading to irreversible capacity loss. Increased metal-oxygen covalence was universally observed in the spectroscopy in the form of a rising pre-O K-edge peak at ~530 eV as a function of lithium deintercalation in both bulk and surface. However, accompanying changes in the Co spectroscopy were only observed in the Fe-substituted sample. These changes are also indicative of increased hybridization between Co 3d and O 2p orbitals. Co K-edge XANES and EXAFS experiments further corroborated these findings, in which virtually no changes in the Co K-edge were observed upon oxidation of unsubstituted LiCoPO4. Fe doping appears to play a substantial role in getting Co to participate in redox chemistry, and the mechanism by which it occurs is currently being explored using Density Functional Theory. Fe-substituted LiCoPO4 is an exciting new positive electrode material that may prove useful in advancing Li-ion battery technology.

High voltage cathode materials show promise for improving the energy density of the state-of-the-art lithium ion battery. One notable cathode material that displays a discharge voltage of ~4.8V, and has the potential to improve the specific energy to ~802Wh/kg, is the olivine-structured LiCoPO4. One significant issue, however, that has plagued LiCoPO4-based systems is the irreversible electrolyte decomposition that occurs at higher voltages. The electrolyte decomposition is further perpetuated by the requirement of a constant voltage charging step (trickle charge) that is necessary due to a lower electronic conductivity of the material and increasing interfacial impedance from the unstable electrolyte. We have previously demonstrated that Fe-doped LiCoPO4 performs significantly better than neat LiCoPO4 by stabilizing the cathode structure and improving the electronic conductivity of the material (1). These structural changes, however, do not solve the problem of perpetual electrolyte decomposition that leads to polarization and loss of cycleable lithium. In order to promote a low resistance passivation layer that prevents electrolyte decomposition, many researchers have incorporated electrolyte additives (2-3). We previously demonstrated that using electrolyte additives improves the performance of doped-LiCoPO4 half cells by forming a protective passivation layer on the cathode (4). The cathode half-cell performance can be somewhat misleading, however, because it does not take into consideration the loss of cycleable lithium due to irreversible redox reactions on the electrodes. The present study serves as a next-step for demonstrating the viability of our doped-LiCoPO4 cathode for the production of high specific energy Li-ion batteries. Full coin cells were fabricated and tested, including a next-generation LiCoPO4-doped cathode (developed by ARL) and a standard graphite anode. Various electrolyte additives were examined with the new cathode to minimize the first cycle irreversible capacity loss, reduce the interfacial impedance, and promote a long cycle life. These additives include tris(hexafluoro-iso­-propyl)phosphate (HFiP), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), fluoroethylene carbonate (FEC), trimethylboroxine (TMB), and other ARL-proprietary additives. These additives significantly improved the performance when compared to the baseline electrolyte. ACKNOWLEDGEMENT The authors wish to express their gratitude to the DOE ABR program for partial financial support. REFERENCES J.L. Allen, T.R. Jow and J. Wolfenstine, J. Power Sources, 196, 8656 (2011). R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall and C. Bruenig, Electrochem. Commun., 28, 20 (2013). M. Hu, J. Wei, L. Xing and Z. Zhou, J. Appl. Electrochem., 42, 291 (2012). J.L Allen, J.L. Allen, S.A. Delp, T.R. Jow, ECS Trans. 27, 63-68 (2014)

There is an ongoing need for higher energy batteries that operate with the highest abuse tolerance. LiCoPO4 is a 4.8 V cathode material with potentially better abuse tolerance and energy density than the current state of art Li-ion battery chemistries. The intense development of high voltage electrolytes1 and strategies that improve the cycle life2, discharge capacity2 and rate capability3 have increased the interest in this material. This paper will review our recent efforts on LiCoPO4 electrode development. We will discuss synthesis methods, transport measurements, thermal properties and the improvements in discharge capacity and cycle life that result from substitutions to LiCoPO4. References 1. J.L Allen, J.L. Allen, S.A. Delp, T.R. Jow, ECS Trans. 27, 63-68 (2014). 2. J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 196, 8656 (2011). 3. J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow and J. Wolfenstine, J. Power Sources 254, 204 (2014). Figure 1

The electrolyte is a critical part of today’s Li-ion batteries. The state-of-the-art Li-ion batteries, which are made of cathodes such as 4.1 V LiCoO2 and 3.4 V LiFePO4 and graphite anode, function well in the state-of-the-art electrolytes made of LiPF6 in carbonate solvent mixtures such as EC and EMC with additive such as VC. The reason for this compatibility is attributed to the facts that the cathodes are within the stability range of the electrolytes and the ability of the electrolyte for forming a solid electrolyte interphase (SEI) layer on graphite, protecting the graphite anode from reacting with the electrolytes. When developing higher energy density Li-ion batteries based on higher voltage cathodes such as 4.7 V LiNi0.5Mn1.5O4 (LNMO) and 4.8 V LiCoPO4 (LCP), the oxidative stability of the state-of-the-art electrolytes is in question. Furthermore, the stability of the high voltage cathodes themselves is also not certain. Efforts in improving the stability of LNMO and LCP through substitutions have yielded better performing cathodes in terms of improved capacity retention and coulombic efficiency.1, 2 The search for electrolytes that work with the high voltage cathodes has been intensive. Molecular modeling to understand the oxidative stability of Li-ion electrolytes has been active.3 This paper will review our recent efforts on electrolyte development for improving the electrochemical performance of the high voltage cathodes in Li-ion batteries. References A. Manthiram, Phys.Chem. Lett. 2011, 2, 176-184. J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 2011, 196, 8656. O. Borodin, W. Behl, T. R. Jow, J. Phys. Chem. C 2013, 117, 8661-8682.

There is an ongoing need for higher energy batteries that operate with the highest safety margin. LiCoPO4 is a 4.8 V cathode material with potentially better abuse tolerance and energy density than the current state of art Li-ion battery chemistries. The intense development of high voltage electrolytes1 and strategies that improve the cycle life2, discharge capacity2 and rate capability3have increased the interest in this material. This paper will review our recent efforts on LiCoPO4 electrode development for improving the energy and abuse tolerance of Li-ion batteries. We will discuss synthesis methods, transport measurements,3 substitutional chemistry of LiCoPO4 and performance enhancements that result from reformulating the electrolyte. References A. V. Cresce, A.V and K. Xu, J. Electrochem. Soc. 158, A337, 2011. J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 2011, 196, 8656. J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow and J. Wolfenstine, J. Power Sources 254, 204 (2014).

Since the electrochemical potential difference and the charge capacity largely determine the energy density of a Li-ion battery, a major and concurrent factor in the choice of electrode materials for high-voltage Li-ion chemistry is the insight gained from studying and understanding the electrochemical reactions which take place at the electrode/electrolyte interface. The extreme electrochemical potential of high-voltage cathode materials marks a limit on the oxidation stability window of the state-of-the-art nonaqueous electrolytes used in Li-ion chemistry. In addition, the properties of the passivation layer for any particular cathode electrode material are determined by the composition of the electrolyte and electrolyte additives. The importance of understanding how the electrode surface reacts with the electrolyte for advancing Li-ion energy storage capacity is evidenced, in part, by kinetics measurements which have demonstrated that the activation energy of Li+ charge transfer rates varies with differing electrode materials. This clearly suggests that the different electrodes create distinct chemical interfaces in the same electrolyte. Spinel dissolution and cathodic capacity losses in metal-oxide cells as a function of varying electrolyte solution setting, comprising different mixtures of solvents, Li salts and additives, is a major area of current research. It has been shown that spinel dissolution is induced by acids resulting from electrochemical oxidation of solvent molecules on composite cathodes. The widely utilized carbonate based solvents are considered to be relatively inert, whereas other related solvents such as various ethers are readily oxidized to produce acids. Experimental charge/discharge cycling of spinel-loaded composite cathodes found the acid concentration and the extent of spinel dissolution to be significantly higher in ether-containing electrolytes in comparison to their carbonate based counterparts. Further experimental evidence indicates that Li and transition-metal ion extraction is facilitated by an acid induced degradation of the cathode material, where continued spinel dissolution leads to oxygen loss from the Li-metal-oxide lattice. The nature of added Li salts largely influences solvent oxidation and spinel dissolution. Although solvent induced acid production for electrolytes containing fluorinated salts is not significant, the appreciable spinel dissolution in these cathode/electrolyte systems strongly suggests that acid is generated through various reaction pathways. A thorough understanding of the reactions on the electrode surface of lithium batteries is central to the designing of new electrode interface material components to achieve efficiency and durability in high-voltage settings. In the present computational modeling work, we focus on the high-voltage lithium nickel manganese spinel (LiNi0.5Mn1.5O4) cathode material, where an experimental investigation of the voltage-dependent electrochemical reactions of a 1-M LiPF6/EC:DMC:DEC electrolyte on a LiNi0.5Mn1.5O4-based electrode, through characterization of the surface species by XPS and FTIR-ATR, showed that the increase in current flow coincides with changes to the surface chemistry of the cathodes and displayed a clear trend of increasing polyethylenecarbonate formation with increasing voltage. In order to provide predictive understanding in complement to experimental work, we performed high-level Quantum Chemical (QC) calculations and DFT-based Born-Oppenheimer Molecular Dynamics (BOMD) simulations on a series of non-aqueous, mainly carbonate-based electrolytes featuring a combination of solvents, salts, electrolyte additives, and cathode materials. Initial BOMD calculations of molecule/surface systems (EC, DEC, FEC, DMC, PF6 -, HFIB) served as a guide for subsequent unconstrained BOMD simulations of electrolyte mixtures on the delithiated LiNi0.5Mn1.5O4 [111] and [100] surfaces to capture spontaneous electrochemical reactions. Nudged Elastic Band and Path Minimization calculations were employed to study the chemisorption characteristics of each electrolyte, such as the internal bond-breaking/bond-making evolution of the electrolyte and the electrolyte coordination to surface metal ions. Then, DFT potential-of-mean-force (PMF) simulations were conducted on explicit liquid electrolyte/electrode interfaces at finite temperature to investigate the previously determined pathways. From the PMF calculations, potential key reaction steps such as the transfer of protons and the lowering of the reaction barrier in the explicit liquid environment were determined. In particular, the two mechanisms of main interest in regard to interfacial electrolyte/cathode chemistry studied in the simulations is the retrieval of an oxygen ion from the cathode surface by an oxidized electrolyte molecular fragment and the proton transfer from the carbonate electrolyte to the metal-oxide. Our DFT based molecular dynamics simulations conducted at liquid EC/cathode interfaces are consistent with the view that reactions and electron transfer occur at the electrode interface.

The electrolyte is a critical part of today’s Li-ion batteries. The state-of-the-art Li-ion batteries, which are made of cathodes such as 4.1 V LiCoO2 and 3.4 V LiFePO4 and graphite anode, function well in the state-of-the-art electrolytes made of LiPF6 in carbonate solvent mixtures such as EC and EMC with additive such as VC. The reason for this compatibility is attributed to the facts that the cathodes are within the stability range of the electrolytes and the ability of the electrolyte for forming a solid electrolyte interphase (SEI) layer on graphite, protecting the graphite anode from reacting with the electrolytes. When developing higher energy density Li-ion batteries based on higher voltage cathodes such as 4.7 V LiNi0.5Mn1.5O4 (LNMO) and 4.8 V LiCoPO4 (LCP), the oxidative stability of the state-of-the-art electrolytes is in question. Furthermore, the stability of the high voltage cathodes themselves is also not certain. Efforts in improving the stability of LNMO and LCP through substitutions have yielded better performing cathodes in terms of improved capacity retention and coulombic efficiency.1, 2 The search for electrolytes that work with the high voltage cathodes has been intensive. Molecular modeling to understand the oxidative stability of Li-ion electrolytes has been active.3 This paper will review our recent efforts on electrolyte development for improving the electrochemical performance of the high voltage cathodes in Li-ion batteries. References A. Manthiram, Phys.Chem. Lett. 2011, 2, 176-184. J. L. Allen, T. R. Jow, J. Wolfenstine, J. Power Sources 2011, 196, 8656. O. Borodin, W. Behl, T. R. Jow, J. Phys. Chem. C 2013, 117, 8661-8682.

Advanced high voltage cathode materials can increase the energy density through increased charging voltage but this usually limits the cell’s cycle life and/or efficiency. This tradeoff is due to irreversible electrolyte decomposition or damage to the cathode structure that occurs at higher potentials. Furthermore, materials such as LiCoPO4 are naturally electronically insulating and thus may either require a constant voltage charging step or very high voltage constant current cutoff voltage( >5.0V) to ensure a full charge is obtained. This can lead to perpetual electrolyte decomposition on each cycle due to the increased time the cell is held at higher potentials. Many researchers have incorporated various transition metals as dopants to improve the cycle life of these cells while maintaining the same cutoff voltage, and subsequently achieving higher energy density and cycle life. This was demonstrated recently in our lab by Fe-doping LiCoPO4 (1). This is perhaps attributed to better electronic conductivity of Fe-doped LiCoPO4and stabilization of the cathode material. An alternative approach involves the use of electrolyte additives for improved passivation layer formation (2-3). This approach, however, does not affect any structural changes that may occur within the cathode (i.e., additives are not as effective in stabilizing the cathode structure as doped metals). The use of additives can nevertheless improve the high-temperature performance of the cell and ensure a longer cycle life (4). This study utilizes a joint cathode/electrolyte research approach. A next-generation LiCoPO4-doped cathode was used (developed by ARL) that is less insulating than traditional LiCoPO4, as indicated by a shorter CV step requirement. Various electrolyte additives were examined with the new cathode to further optimize the performance. These additives include tris(hexafluoro-iso-propyl)phosphate (HFiP), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), fluoroethylene carbonate (FEC) and Trimethylboroxine (TMB). While investigating the properties of the new cathode material, the electrolyte mixture was optimized for high-capacity cycling. This study has yielded a high-efficiency cell with a long cycle-life that is stable at room temperature and elevated temperatures. ACKNOWLEDGEMENT The authors wish to express their gratitude to the DOE ABR program for partial financial support. REFERENCES J.L. Allen, T.R. Jow and J. Wolfenstine, J. Power Sources, 196, 8656 (2011). R. Sharabi, E. Markevich, K. Fridman, G. Gershinsky, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall and C. Bruenig, Electrochem. Commun., 28, 20 (2013). M. Hu, J. Wei, L. Xing and Z. Zhou, J. Appl. Electrochem., 42, 291 (2012). N.P.W. Pieczonka, L. Yang, M.P. Balogh, B.R. Powell, K. Chemelewski, A. Manthiram, S.A. Krachkovskiy, G.R. Goward, M. Liu and J.-H. Kim, J. Phys. Chem. C, 117, 22603 (2013).

The approach of replacing the state-of-the-art Li-ion battery cathodes such as 4.2 V lithium cobalt oxide LiCoO2 and lithium nickel cobalt manganese oxide LiNi1/3Co1/3Mn1/3O2 with high voltage cathodes such as 4.7 V lithium nickel manganese spinel LiMn1.5Ni0.5O4 (LNMO) and 4.8 V lithium cobalt phosphate LiCoPO4 (LCP) [1], versus Li/Li+, for the increase of energy density has encountered problems including lower Coulombic efficiency (CE) [2-4], faster capacity fading [5], and therefore, shorter cycle life than that of the state-of-the-art Li-ion batteries have achieved. At elevated temperatures, e.g. 55 oC, the LNMO/graphite or LCP/graphite cells when cycled in the state-of-the-art Li-ion electrolytes, e.g. 1.2 M LiPF6 in EC:EMC (3:7 w/o), shows CE values can be as low as below 90% [4]. What are the issues? Electrolyte degradation at high voltages is one. The instability of the cathode at high voltage is another one, which may include the instability of cathode structure itself and the high cathode reactivity with the electrolytes. Additionally, the degradation at the cathode side also plays a role in degrading the anode resulting in poor cell performance. The understanding of electrolyte oxidative stability at high voltages through computation [6] and surface analysis of reaction products on the surfaces of both cathode and anode [7] indicates that very much improved electrolytes are urgently needed. The modification of the electronic structures of LNMO and LCP through substitutions [4,5] also improves capacity retention. This suggests that the stability of the high voltage cathodes can be improved. The developments of improved electrolytes including the use of additives and fluorinated solvents for improved electrolyte stability and improved LNMO and LCP cathodes using substitution for stabilizing the cathodes will be reviewed. Recent advances coupling the improvements in high voltage cathode materials and electrolytes together for better performance will also be presented. ACKNOWLEDGEMENT The authors wish to express their gratitude to the DOE ABR program for partial financial support. REFERENCES A. Manthiram, Materials Challenges and Opportunities of Lithium Ion Batteries, Phys. Chem. Lett., 2011, 2, 176-184. H. Duncan, D. Duguay, Y. Abu-Lebdeh, I. J. Davidson, “Study of the LiMn1.5Ni0.5O4/Electrolyte Interface at Room Temperature and 60 oC,” J. Electrochem. Soc., 2011, 158 (5), A537-A545. J. Wolfenstine, U. Lee, B. Poese and J. L. Allen, “Effect of oxygen partial pressure on the discharge capacity of LiCoPO4,”J. Power Sources, 2005 144 226. D. W. Shin, C. A. Bridges, A. Huq, M. P. Paranthaman, A. Manthiram, Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries, Chem. Mater., 2012, 24, 3720-3731. J. L. Allen, T. R. Jow, J. Wolfenstine, “Improved Cycle Life of Fe-substituted LiCoPO4,” J. Power Sources, 2011, 196, 8656. O. Borodin, W. Behl, and T. R. Jow, “Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone and Alkyl Phosphate-Based Electrolytes,” J. Phys. Chem. C, 2013, 117, 8661-8682. D. Lu, M. Xu, L. Zhou, A. Garsuch, B. L. Lucht, “Failure Mechanism of Graphite/LiNi0.5Mn1.5O4 Cells at High Voltage and Elevated Temperature,” J. Electrochem. Soc., 2013, 160 (5), A3138-A3143.

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Fe-substituted LiCoPO4 exhibits greatly improved cycle life relative to LiCoPO4. Whereas, pure LiCoPO4 loses more than half of its discharge capacity at the 10th cycle, the Fe-substituted LiCoPO4 retains about 100% of its discharge capacity at the 10th cycle and about 80% of its capacity at the 500th cycle. It is suggested that improved cycle life results from Fe3+ substitution on the Li and Co sites. The partial substitution of Li+ by Fe3+ and Co2+ by Fe2+ and Fe3+ was evidenced from Rietveld analysis of X-ray powder diffraction data, infrared spectroscopy, X-ray photoelectron spectroscopy and Mössbauer spectroscopy. The majority of the Fe3+ substitutes at the Co2+ site. The composition of Fe-substituted LiCoPO4 is Li0.92Co0.8Fe2+0.12Fe3+0.08PO4 for a sample of starting composition LiCo0.8Fe0.2PO4.

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