Finkelstein Sam H, Ricci Marco, Bötticher Tom, Schmidt-Rohr Klaus
Department of Chemistry, Brandeis University, Waltham, MA 02453, USA.
Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy.
Phys Chem Chem Phys. 2024 Sep 18;26(36):24157-24171. doi: 10.1039/d4cp00818a.
A good explanation of lithium-ion batteries (LIBs) needs to convincingly account for the spontaneous, energy-releasing movement of lithium ions and electrons out of the negative and into the positive electrode, the defining characteristic of working LIBs. We analyze a discharging battery with a two-phase LiFePO/FePO positive electrode (cathode) from a thermodynamic perspective and show that, compared to loosely-bound lithium in the negative electrode (anode), lithium in the ionic positive electrode is more strongly bonded, moves there in an energetically downhill irreversible process, and ends up trapped in the positive electrode. Only a sufficiently high charging voltage can drive it back to the other electrode. Since the stronger bonding in the positive electrode lowers the energy by ∼320 kJ mol, a lot of energy is released. This explanation is quantitatively supported by an analysis of cohesive-energy differences of the electrode materials. Since electrons are only intermediates in the discharge reaction and the chemical potential of the electron cannot be measured, electrons do not need to be assigned a distinct energetic role. The incorporation of Li and an electron into the cathode is accompanied by the reduction of another ion or atom, usually a transition metal such as Fe or Co. The metal's ionization energy in the corresponding oxidation step correlates with the cell voltage, based on a decomposition of cohesive energy into electronic and ionic components. We relate the differences in cohesive energies to the chemical potential of lithium atoms, which is quantified, for instance for a two-phase electrode. The analysis is extended to a single-phase LiCoO cathode, whose average voltage can be calculated from the cohesive-energy difference between LiCoO and CoO.
对锂离子电池（LIBs）的一个合理解释需要令人信服地说明锂离子和电子自发地从负极移动到正极并释放能量这一工作锂离子电池的决定性特征。我们从热力学角度分析了一个具有两相LiFePO₄/FePO₄正极（阴极）的放电电池，结果表明，与负极（阳极）中结合松散的锂相比，离子正极中的锂结合更强，在能量上以不可逆的下坡过程移动到那里，并最终被困在正极中。只有足够高的充电电压才能将其驱动回另一电极。由于正极中更强的结合使能量降低了约320 kJ/mol，因此释放了大量能量。电极材料内聚能差异的分析在定量上支持了这一解释。由于电子在放电反应中只是中间体，且电子的化学势无法测量，因此无需为电子赋予独特的能量作用。锂和一个电子掺入阴极伴随着另一种离子或原子的还原，通常是过渡金属，如铁或钴。基于内聚能分解为电子和离子成分，金属在相应氧化步骤中的电离能与电池电压相关。我们将内聚能的差异与锂原子的化学势联系起来，例如对于两相电极，锂原子的化学势是可以量化的。该分析扩展到单相LiCoO₂阴极，其平均电压可以根据LiCoO₂和CoO之间的内聚能差异计算得出。
