Abstract
Lithium ion batteries are widely used in portable electronics and transportations due to their high energy and high power with low cost. However, they suffer from capacity degradation during long cycling, thus making it urgent to study their decay mechanisms. Commercial 18650-type LiCoO2 + LiNi0.5Mn0.3Co0.2O2/graphite cells are cycled at 1 C rate for 700 cycles, and a continuous post-mortem analysis is performed. Based on these tests, the decay mechanism of the cells is finally proposed. The changes of differential capacity curves of the full cells show that the loss of active materials, loss of lithium ions and cell polarization are the main factors contributing to capacity loss. Non-fully charging of the electrodes is also one of the reasons, but only takes up a minor portion. Impedance results indicate that the charge transfer resistance becomes larger during cycling, especially at low state of charge. Morphology and surface chemistry analysis demonstrates that the electrode particles after cycling exhibit some minor cracks and some additional layers are formed on surfaces of both the cathode and anode electrodes. All of these effects may contribute to the impedance increase, and consequently lead to degradation of the full cells. Thus, a good protection of the surface of the cathode and anode shows great potential to improve the capacity maintenance and prolong the cycle life of the cells.
Highlights
In the past few decades, the demand of lithium ion batteries (LIBs) with high energy density and low cost, has been increasing as power sources for portable electronics, electric vehicles (EVs) and renewable energy storages [1,2,3,4,5]
Among the commercial cathodes of lithium ion batteries, lithium cobalt oxide (LCO) is one of the earliest materials widely used in portable electronics, due to its high energy density [7,8]
Continuous electrochemical changes can be determined through analyzing the differential capacity curves of cycled cells with different cycle numbers
Summary
In the past few decades, the demand of lithium ion batteries (LIBs) with high energy density and low cost, has been increasing as power sources for portable electronics, electric vehicles (EVs) and renewable energy storages [1,2,3,4,5]. Energies 2017, 10, 1147 people, the lithium ion batteries using LCO + NMC532 blended cathodes combined with Gr anode are becoming one of the most promising cell chemistries, due to their high energy density, power density, safety performance and moderate cost. Despite such advantages, it suffers from capacity degradation with long cycling during the usage process. Kobayashi et al [23] studied the fading mechanism of cylindrical nickel-cobalt oxide (LiNi0.73 Co0.17 Al0.10 O2 )/hard carbon lithium ion batteries They revealed the crystal structure of the positive electrode changed from rhombohedral to cubic symmetry on the surface and a Li2 CO3 film was formed after cycling. Were discussed and the strategies to improve their cycling performance were suggested
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