In situ synchrotron diffraction revealed a stepwise appearance of two new phases upon electrochemical lithium extraction from LiCoPO 4 . These phases were demonstrated to have the same olivine-like structure as the pristine compound. The lithium-deficient phases were proposed to be Li 0.7 CoPO 4 and CoPO 4 . The completely delithiated phase appears to be unstable in air and undergoes amorphization. The phase transitions are reversible, but a slow kinetics of the initial delithiation was identified by in situ synchrotron diffraction and the potentiostatic intermittent titration technique. We demonstrated that the electrochemical extraction of lithium is accompanied by significant electrolyte decomposition, contributing to the capacity loss upon cycling. The galvanostatic intermittent titration technique combined with impedance spectroscopy revealed self-discharge of the cell in the charged state. This study argues different mechanisms of lithium extraction from LiCoPO 4 in comparison with LiFePO 4 and LiMnPO 4 .

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The electrochemical performance of LiCoPO 4 -based electrodes was studied in dependence on synthetic conditions. The role of particle size distribution is illustrated. Structural changes occurring at lithium extraction-insertion was explored by ex-situ and in-situ synchrotron XRD measurements. For the samples annealed at temperature lower than 600°C the amorphisation of the cathode material was identified. For a high-temperature sample the two-phase character of electrochemical reaction was confirmed. The character of structural changes observed at charge and discharge is different: the two-phase reaction LiCoPO 4 -"CoPO 4 " occurs during charging, while discharge induces a strong increase of the cell volume for the Li-poor phase. KeywordsPhospho-olivine, lithium batteries, in situ diffraction, synchrotron * Corresponding author of lithium deinsertion from LiMnPO 4 and LiCoPO 4 seems to take place also [9][10][11][12]. According to Li et al., the second phase arising by the lithium extraction from lithium manganese phosphate has a composition Li x MnPO 4 , where x is very close to zero [9]. Recently, Delacourt et al. showed that the electrochemical delithiation of LiMnPO 4 , prepared by a new precipitation route, leads to the disappearing of pristine LiMnPO 4 and to the formation of the new phase with the same structure, but with lower cell volume (302 Å 3 and 275 Å 3 , respectively) [10]. The authors suggest that the appearing phase is MnPO 4 .Previously, we have shown that LiCoPO 4 -samples prepared by solid-state reaction at high temperature demonstrate unsatisfactory electrochemical performance [13]. The reversibility of

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