Study On Cycling Degradation Mechanism Of Li-Rich And Mn-Based Layered Oxide/Graphite-SiO_x Battery

Li-rich and Mn-based layered oxide cathode material(Li1+xM1-xO2 or xLi2MnO3-(1x)LiMO2(M=Ni,Co,Mn,0<x<1))cathode material has the advantages of high-specific capacity,high working voltage,and low cost.The high specific energy Li-ion battery prepared by matching it with Graphite-SiOx anode material with high capacity is expected to solve the "range anxiety" problem of electric vehicles.However,this kind of battery also has the disadvantage of rapid decay of cycling capacity.It is essential to study the capacity degradation mechanism during cycling to guide the improvement of cycling performance.In this paper,we investigated the cycling capacity decay characteristics of Li1.13Ni0.30Mn0.56Co0.01O2/Graphite-SiOx battery by non-destructive electrochemical and disassembly analysis at different charging cut-off voltages and different charging protocols,and new insights into its capacity degradation mechanism were proposed.Firstly,the capacity decay characteristics and degradation mechanism of the Li1.13Ni0.30Mn0.56Co0.01O2/Graphite-SiOx battery using the constant current(CC)charging protocol in three charge/discharge intervals of 2.0-4.4 V,2.0-4.5 V,and 2.0-4.6 V were investigated.The research shows that the capacity decay rate of the Li1.13Ni0.30Mn0.56Co0.01O2/Graphite-SiOx battery increases with increasing charge cutoff voltage and then tends to be stable.Non-destructive electrochemical and post-mortem analysis showed that increased side reactions on the anode surface during the first 50 cycles led to increased reversible lithium consumption.At this time,the continuous consumption of reversible lithium by solid electrolyte interface(SEI)film was the main reason for the accelerated degradation of the battery;After 50 cycles,with the increase of cut-off voltage,the content of LiF,Li2O,and other inorganic components in the SEI film increases,which improves the stability of the SEI film,and further inhibits the rate of capacity decay to a certain extent.It is clarified that the growth of cut-off voltage will cause two different effects of reversible lithium consumption and the growth of inorganic components in SEI film,which together determine the capacity decay rate of the battery.Secondly,the capacity degradation mechanism of the Li1.13Ni0.30Mn0.56Co0.01O2/Graphite-SiOx battery with constant current/constant voltage(CC-CV)charging protocol in two charge/discharge intervals,2.0-4.5 V and 2.0-4.6 V were investigated.It was found that the capacity declines rapidly at 4.6 V under this charging protocol,and the cycle performance of the battery under 4.5 V is better than that under the CC charging protocol.The effects of the CV process at different cut-off voltages were revealed by non-destructive electrochemical and post-mortem analysis.When 4.5 V cut-off voltage is used,the CV process makes the content of Li2O,LiF,and other inorganic components in the SEI film on the anode surface higher,the SEI film structure more stable,slows down the consumption of reversible lithium.At 4.6 V cut-off voltage,the loss of the anode is relatively serious,Monitoring with the developed high stability graphite micro reference electrode shows that the anode loss dominates after 50 cycles,resulting in the voltage window of cathode and anode shifting to lower voltage after 50 cycles,and lithium deposition occurs after 200 cycles,which makes the reversible lithium continuously consumed and the impedance increases rapidly;at the same time,the cathode structure degradation is greater,the electrolyte side reaction is more serious,and the battery capacity decay rate increases significantly.Finally,the capacity degradation mechanism of the Li1.13Ni0.30Mn0.56Co0.01O2/GraphiteSiOx battery at 45℃(CC-CV charging protocol and 4.5 V cut-off voltage)was explored.It was found that the battery capacity at 45℃ decreased rapidly and continuously.Combined with the post-mortem analysis,it was shown that compared with the performance degradation of the cathode at the two temperatures after cycling,the capacity degradation of the anode at 45℃ was more obvious.XPS,SEM,and TEM studies showed that the dissolution and deposition of transition metal ions intensified under high-temperature condition,resulting in instability of the SEI film,and leading to increased reversible lithium consumption.Three electrode battery tests showed that the anode loss at 45℃ after 200 cycles plays a leading role in the capacity loss,causing the voltage window of the cathode and anode to shift to low voltage.In addition,GC and GC-MS test show intensified reduction and decomposition of the electrolyte at high temperatures,which further increased the reversible lithium consumption and impedance,leading to the continuous decrease in cycling capacity.By studying the capacity decay characteristics of Li1.13Ni0.30Mn0.56-Co0.01O2/GraphiteSiOx battery under different cycling conditions,this paper puts forward new ideas on its capacity attenuation mechanism,indicating that a suitable charging protocol can help improve the stability of the SEI film and facilitate cycling performance.Meanwhile,the improvement of battery performance not only needs to improve the stability of SEI film,but also needs to improve the stability of Graphite-SiOx anode.This paper provides a key idea for improving the cycle performance of Li-rich and Mn-based layered oxide/Graphite-SiOx high energy density battery,and has strong theoretical and practical significance.