Advancements in Lithium-Based Batteries : Unraveling and Mitigating Performance-Limiting Phenomena in Negative Electrodes

Sammanfattning: The development of lithium-based batteries, especially lithium-ion batteries, has changed our daily lives significantly. The technologies first enabled by lithium-based batteries are now in turn creating a demand for better lithium-based batteries with higher energy densities and longer cycling lifetimes. This requires studies and development of negative electrodes, with a particular focus on gaining a better and more complete understanding of all performance-limiting phenomena and to develop appropriate mitigation strategies.For negative electrodes based on alloy-forming materials such as silicon, aluminum and tin, it has, recently been shown that capacities losses can result from diffusion-controlled trapping of lithium due to an incomplete delithiation of the electrodes. In the first part of the thesis, it is demonstrated explicitly that such a lithium trapping effect also is seen for conventional graphite electrodes. This effect is further demonstrated to greatly affect the cycle life performance of NMC811/graphite full cells during high-rate cycling. However, it is also demonstrated that the inclusion of constant-voltage delithiation steps can increase the delithiation efficiency and decrease the influence of the trapping effect. The use of silicon electrodes based on silicon nanoparticles is also revisited. It is proposed that the influence of the lithium trapping effect seen for such electrodes is affected by the size of the employed silicon nanoparticles, most likely, via its influence on the electrode microstructure. Replacing the currently used negative electrodes with lithium-metal electrodes, especially in a “anode-free” configuration, can significantly increase the energy densities of lithium-based batteries due to the high capacity and low electrochemical potential of elemental lithium. However, the inhomogeneous lithium deposition and stripping greatly limit the cycling performance. In the second part of this thesis, strategies focusing on electrochemically controlling the nucleation and growth of lithium are proposed to improve the deposition of lithium on lithium-metal electrodes as well as directly on copper current collectors. In the former study, it is shown that forming a great number of homogeneously distributed nucleation sites across the entire electrode surface, via the introduction of a one-second long potentiostatic oxidation pulse, subsequently yields more homogeneous lithium deposition. In the second study, it is found that the nucleation of lithium on copper current collectors can be affected by the diffusion of lithium into the current collectors. It is also demonstrated that the influence of this effect can be decreased by chemically prelithiating the current collectors so that more homogeneous lithium deposition can be attained.