liquid materials: organic electrolyte (containing lithium salts, solvents and other additives), etc.
II. Lithium-ion batteries work on the basis of two basic processes: the mass transfer process of charged particles and the electrode chemical reaction.
Both materials and processes in lithium-ion batteries are affected by temperature
Solid materials cannot escape the shackles of “thermal expansion and contraction” (ions are neither easy to embed nor easy to get out, and it is difficult to get through the diaphragm); liquid materials cannot escape the fate of increasing viscosity or even solidification at low temperatures (ions “can’t run”); the mass transfer process of charged particles and the electrode The speed of mass transfer and electrode chemical reactions of charged particles is inevitably reduced. The components of a lithium-ion battery are also less compatible at low temperatures. This is why lithium-ion batteries are so “vulnerable” at low temperatures. A “cold” lithium-ion battery will work with greater resistance (higher resistance) and will work less efficiently (rapid drop in actual capacity), and if pushed too hard (high current charging and discharging), the resistance will become greater and the capacity will drop even faster.
III. Low-temperature ageing of lithium-ion batteries results in irreversible capacity loss
Lithium-ion batteries are fear the cold, which means that low temperatures not only reduce the efficiency of lithium-ion batteries but also cause more or less damage to the materials used in lithium-ion batteries. The “irreversible damage” in the electrode chemical reactions that are considered reversible within the battery can be divided into irreversible structural damage to the materials and permanent loss of active material (especially recycled lithium).
1). We may have a question: Does low temperature cause irreversible losses in lithium-ion batteries during non-use?
There are two main ageing mechanisms for lithium-ion batteries: calendar ageing and cyclic ageing. Calendar ageing is the ageing during static non-use storage. It is mainly influenced by temperature and SOC (how much lithium ions are stored in the negative graphite): at high temperatures and high SOC, the electrode/electrolyte interface becomes less stable and side reactions increase – the dissolution of positive metal ions, precipitation of oxygen, decomposition of the electrolyte and thickening of the SEI film on the negative electrode surface. Therefore, low temperatures can in a way inhibit the calendar ageing process. That is, during periods of non-use, if mechanical damage due to cold stress (thermal expansion and contraction) is not discussed, low-temperature conditions, by themselves, do not cause irreversible losses in lithium-ion batteries. In other words, the ageing of lithium-ion batteries at low temperatures is mainly due to cyclic ageing caused by dynamic charge and discharge processes
2). Low-temperature cyclic ageing mainly comes from (a) the growth of lithium plating and lithium dendrites; (b) thickening of SEI; (c) local lattice disruption of the electrode material and (d) popularization decomposition of the electrolyte.
(a) Lithium plating and growth of lithium dendrites. During the charging process, the low temperature causes the lattice to contract, leaving insufficient space for lithium to be embedded in the negative electrode, and charge transfer and solid-phase diffusion become slower. The lithium ions that cannot be embedded in the negative electrode can only gain electrons on the surface of the negative electrode, resulting in the formation of silver-white lithium metal monomers, which is the behaviour of lithium plating (lithium precipitation). The in-homogeneous growth of low-temperature lithium plating can easily form lithium dendrites, and large lithium dendrites can puncture the diaphragm and even cause functional failure. During discharge, the reaction rate between the lithium metal deposited on the surface of the negative electrode and the electrolyte also decreases, and the closer the lithium monomers are to the collector, the more they will be the first to dissolve, leaving the top lithium unconnected to the negative electrode, resulting in “dead lithium”, which will be a permanent and irreversible loss.
(b) Thickening of the SEI film. The lithiation potential of the anode material in lithium-ion batteries is often lower than the reduction decomposition potential of the organic electrolyte and therefore a passivation layer, the SEI film, is formed throughout the life of the battery. The rise in resistance leads to greater polarization, a shift to lower potentials, and the act of lithium plating means that the potential is lower, so the decomposition of the organic electrolyte will “follow” the lithium plating, forming thicker and thicker SEI films, with circulating lithium being lost in the process and increasing resistance. Therefore, many studies have focused on low-temperature charging strategies, suggesting lower current densities to allow the lithium-embedding behaviour to “take its time”.
(c) Local lattice damage of the electrode material. The contraction of the lattice at low temperatures, which is strongly embedded, can easily lead to local lattice damage within the positive and negative electrode materials, which cannot be repaired by itself.
(d) Polarization decomposition of the electrolyte. Under low-temperature conditions, electrochemical polarization and concentration polarisation are severe and side reactions can easily occur at the electrode/electrolyte interface, leading to the decomposition of the electrolyte; also the decomposition of the organic electrolyte is irreversibly damaged during the thickening of the SEI film.
Ⅳ Are solid electrolytes capable of reversing irreversible losses resulting from low temperatures?
In all-solid-state lithium-ion batteries, solid-state electrolytes have strong mechanical properties that effectively inhibit the growth of lithium dendrites, especially in all-solid-state thin-film lithium-ion batteries, which do not require the addition of conductive agents and binders and have fewer mechanisms to cause deterioration in low-temperature performance.
However, existing solid-state electrolytes still present significant problems in constructing a good contact “electrode/electrolyte interface”.
The solid-solid interface is often hardly as compatible as the solid-liquid interface, while the problems of lithium plating and lithium dendrite growth remain to a greater or lesser extent.
High interfacial resistance, where interfacial chemical reactions remain, leading to decomposition of electrolyte components and loss of recycled lithium.
The ionic conductivity of the solid electrolyte is lower than that of the liquid organic electrolyte. At low temperatures, there is still a reduction in ion transport rate and an increase in resistance.
Local structural damage within the electrode material is not fundamentally transformed by changes in the electrolyte. Charging or discharging at low temperatures has an irreversible effect on the lithium-ion battery, resulting in a dive in capacity and a serious safety hazard. Prolonged storage at ultra-low temperatures (-20℃) also has an irreversible effect on the battery, reducing its capacity. Therefore we should care about the lithium-ion battery use environment temperature or take some heating strategy for it. Charging or discharging at low temperatures has an irreversible effect on the lithium-ion battery, resulting in a dive in capacity and a serious safety hazard. Prolonged storage at ultra-low temperatures (-20℃) also has an irreversible effect on the battery, reducing its capacity. Therefore we should care about the lithium-ion battery use environment temperature or take some heating strategy for it.