Electrochemical Stiffness in Anodes and Cathodes for Lithium Ion Batteries

Higher power and charging rates are essential for large scale adoption of lithium-ion batteries in transportation applications. While lithium-ion batteries are ubiquitous in portable electronics, their performance and lifetime suffer during high rate charging and high power discharging. In such extreme electrochemical conditions, the mechanical response of electrode materials governs their degradation behavior. Here, we present a new technique to probe the electrochemically-induced mechanics of electrodes by calculating the electrochemical stiffness of electrodes via coordinated in situ stress and strain measurements.  Through the electrochemical stiffness, we elucidate inherent and rate-dependent mechanical responses of graphitic battery electrodes. We find that stress and strain are asynchronous as shown in the figure.  In particular, stress development is found to lead strain development as different graphite-lithium intercalation compounds are formed. Additionally, our analysis reveals inversely scaled rate-dependent behaviors for stress and strain responses. Stress scales as scan rate while strain scales with charge (or, equivalently, capacity).  These measurements provide a new paradigm for understanding mechanical effects in intercalation systems, such as batteries.  Stress arises as resistance to lithiation, while strain arises as a consequence of lithiation.  Electrochemical stiffness measurements provide new insights into the origin of the rate-dependent chemo-mechanical degradations, and provide a probe to evaluate advanced battery electrodes.

We also report on our efforts to stabilize LiMn₂O₄ (LMO)  -- a low cost cathode material for a Li-ion battery  These cathodes suffer from significant capacity fade with cycling.  This capacity fade is in part associated with the dissolution of Mn from the cathode material. To prevent this dissolution, we coated LMO particles with a Au shell by a simple and scalable electroless deposition.  Characterization by SEM, TEM, EELS, and AFM showed that the Au shell was approximately 3 nm thick.  The Au shell prevented much of the Mn from dissolving in the electrolyte with 82% and 88% less dissolved Mn in the electrolyte at room temperature and 65 ºC, respectively, as compared to the uncoated LMO.  These Au coated cathode materials show great improvement in reducing Mn dissolution and extending electrochemical performance through hundreds of cycles over uncoated LMO.