The performance of Li-ion battery is greatly affected by external temperature conditions. Under low temperature conditions, the diffusion kinetics of Li + deteriorates, which leads to the decrease of performance. For example, the discharge capacity of 1C / graphite battery in NMC / graphite 18650 decreases by 20% at -10 ℃, The discharge capacity of 1C decreases by 30% at -20 ° C. The effect of low temperature on the performance of Li-ion battery is mainly reflected in the diffusion rate of Li +, for example, the diffusion speed of Li + is slow at low temperature and may not be complete if the charging current is too large Embedded in the negative electrode, which formed in its surface Li plating, resulting in rapid decline in lithium-ion battery capacity and safety issues arise, the same low-temperature lithium-ion battery discharge rate performance will be greatly affected.
In order to improve the performance of Li-ion battery, its kinetic properties at different temperatures need to be studied in depth. In the past, the confinement of research conditions restricted the real-time reaction of lithium-ion battery In recent years, with the development of in-situ detection technology, especially the more sensitive in situ neutron diffraction technology for Li, we provide a very powerful tool for studying the internal reaction mechanism of Li-ion batteries.
Recently, VeronikaZinth, Christian von Lüders and others at Munich University of Technology in Germany studied the phase transformation of inhomogeneous graphite in negative electrode of lithium ion batteries by in-situ neutron diffraction, revealing different discharge rates and Ambient temperature on the temperature of graphite negative impact on objects.
In the experiment VeronikaZinth used commercial NMC / graphite 18650 battery as the research object, the battery capacity is about 1950mAh / g at 25 ℃ .The lower figure a shows the change of negative pole phase at C / 2 rate discharge at room temperature.From the neutron diffraction pattern As can be seen, the main phase of the graphite negative electrode is LiC6, and a small amount of LiC12, when the battery is fully charged, the ratio of lithium insertion to graphite is 91%. During discharge, with the prolapse of Li, The proportion of LiC6 gradually decreased and the proportion of LiC12 increased gradually.When the discharge depth reached about 40%, the diffraction peak of LiC12 gradually moved toward smaller direction, and the other two phases with lower Li content appeared. Li1-XC18 And Li1-XC54. The lower graph b shows the change of negative phase when the discharge rate is 1C. As can be seen from the graph b, the Li content in the negative electrode is relatively high due to the lower discharge capacity of the lower cell. Therefore, the diffraction of LiC12 The peak does not shift a lot to the left from the above data can be seen, under different C / 2 and 1C rate fully discharged, the graphite anode have appeared a variety of mixed phase phenomenon.
The graph below shows the phase change of the graphite negative electrode during the entire discharging process. As can be seen from the graph, during the discharging process, the concentration of LiC6 gradually decreases to LiC12, and then other lower Li phases begin From the figure we also noticed the discharge rate on graphite negative pole phase at 1C rate, the negative LiC12 concentration should always be lower than the C / 2 ratio, while low Li concentration Li1-XC18 and Li1-XC54 , Also appeared earlier at 1 C discharge. This indicates that there is a large inhomogeneity and a lithium concentration gradient in the graphite negative electrode during rapid discharge of large current.
In general, this electrode heterogeneity, can be properly placed aside, so that Li spread within the graphite to reduce non-uniformity.The following figure shows the phase change of the negative electrode during the battery is placed after the discharge ends, you can It is still very large to see the phase change during the suspension process, especially the battery which is put aside after the 1C rate discharge.The process of the most change mainly occurs in the first 10 min, the main change process is at 20 min (C / 2 ratio) and 40 min 1C ratio) at the discharge rate of 1C battery cathode diffraction peak at d = 3.4731, the corresponding negative electrode compound is LiC18-LiC36 or LiC26-LiC36, or a mixture of both phases, while the C / 2 ratio discharge The diffraction peak of the battery d = about 3.4340, the corresponding negative compounds LiC36-LiC72 and LiC54-LiC85.
The graph below shows the phase change of graphite negative electrode under different discharge magnifications. It can be seen from the figure that when the velocity Ve of Li + prolapse is less than the diffusion velocity Vd of Li + within graphite (small rate discharge), graphite The negative electrode should always be in equilibrium. Therefore, diffraction peaks of single phase and two phases coexist should be observed in the diffraction pattern. If Ve is larger than Vd, a concentration gradient of Li is formed inside the graphite particle and the interior of the particle will form rich Lithium core, outside will form a lithium-poor shell, all of these phases will be observed at the same time, and only be fully shelved to disappear visible in the above experiments, C / 2 and 1C rate graphite negative electrode is The coexistence of multiple phases proves that the release rate of Li + should be higher than the diffusion rate of Li + in graphite, so it needs some time to stand after discharging to reach the equilibrium inside the particles.
The graph below shows the phase change of the negative electrode during discharge at C / 10 at -20 ° C. It can be seen that four different phases (LiC6, LiC12, Li1-XC18 and Li1-XC54) appear at 0.71 Ah Which indicates that the non-uniform phenomenon in the negative electrode of graphite is more serious than that at normal temperature at low temperature.At the same time, the capacity of the battery at low temperature is tested at 1505.8mAh (C / 10 contains a constant voltage discharge process, with a capacity of only 1209.6mAh / g ) Was significantly lower than at room temperature 2002.6mAh (C / 30 rate constant current and constant discharge) capacity.
The graph below shows the phase change of the graphite negative electrode during the shelf process after discharge at low temperature. It can be seen from the graph that the phase change of the graphite negative electrode is obviously slower than that at room temperature due to the deterioration of the kinetic conditions at low temperature. It can be seen that although the lithium negative electrode has not reached an equilibrium even after 11 hours of storage, if the lithium ion battery is not used at low temperature for a long time, the capacity of the lithium ion battery may be decreased.
The performance of Li-ion batteries is greatly influenced by the kinetic conditions. The study by Veronika Zinth shows that both the C / 2 and 1C discharge rates have exceeded the diffusion rate of Li in the graphite. Therefore, during discharge, graphite Negative Li concentration gradient generated within the graphite anode and produce a variety of phases, you must go through a period of shelving in order to ensure the balance within the graphite anode.Low temperature conditions, lithium ion kinetics conditions worse, so the discharge process Graphite anode imbalance is more serious, discharge time should also be shelved to extend, so as to ensure the balance within the negative graphite.
