Influence of temperature and upper cut-off voltage on the formation of lithium-ion cells

The influences of temperature on the formation losses and subsequent electrical performance of Li-x(Ni1/3Co1/3Mn1/3)(y)O-2 (NCM)/graphite lithium-ion cells were investigated. It was shown that the total capacity loss during formation of a full cell at 25 degrees C consists of losses on the positive and negative electrode to about one half each. The losses of the negative electrode (ca. 10%) are due to solid electrolyte interphase (SEI) formation on graphite but are masked by the losses of the positive side (ca. 20%) which are mainly caused by a kinetic inhibition of NCM and are theoretically reversible when the cell is discharged to very low potentials. The total loss of a full cell fits with the loss of the positive electrode. With increased temperature the ratio of losses on positive and negative electrode decreases as the diffusion coefficient of lithium in NCM increases. In total, an elevated formation temperature leads to increased irreversible losses on both electrodes and significantly lower cell performance of graphite. The upper cut-off voltage has an influence on the positive electrode formation losses in a reversible manner. The constituents of the SEI identified via the combination of XPS and FTIR are mainly RCH2OCO2Li, RCOOLi and LiF for the outer SEI and mainly Li2CO3, Li2O and LiF for the inner SEI. (C) 2014 Elsevier B.V. All rights reserved.