Navigating the Impact of Cold on Electric Vehicle Batteries

Cold weather presents several challenges for electric car owners, affecting their range, battery performance, and charging speed. Cold weather temporarily reduces EV battery range, and in a deep freeze, EV owners are dealing with reduced battery performance and increased charging times.

Given that the battery is the most expensive component of an EV and its production contributes significantly to CO2 emissions, maintaining battery health is crucial. If an EV is charged in cold conditions without preheating the battery to an optimal temperature, the process not only takes longer but can also shorten the battery's lifespan. A prematurely aged battery fails to offset the CO2 emissions and replacing it will be very costly for the owner. 

It is considered that Lithium-ion batteries (LIBs), used in EVs, operate best within a temperature range of 15 to 35°C. Outside this range, particularly in subzero temperatures, the internal electrochemical reaction of the battery slows down, internal resistance increases, and its available capacity and energy output significantly decrease.

Research conducted by Chen Zhao and colleagues involved testing six electric vehicles from three Original Equipment Manufacturers on a chassis dynamometer. At first, tests were conducted at an ambient temperature of 25°C, then repeated at −7°C. The results showed that the driving range decreased dramatically under cold temperatures and the average rate of reduction was above 40%, illustrated in the figure below. Market survey has also indicated that the range reduction under cold temperatures is one of the most concerning issues of the customers. That is why it is important to try to increase range reduction in cold temperatures.

At low temperatures, the overall performance of traction batteries experiences a significant decline due to a reduced electrochemical reaction rate and accelerated health degradation, such as lithium plating. Electric vehicles face operational difficulties and safety hazards if timely and effective actions are not taken to address performance degradation. In cold regions, it is crucial to warm up/preheat the battery to mitigate these issues.

Battery Composition and Charging Dynamics

A standard automotive Li-ion battery comprises a cathode, an anode (typically made of graphite), an electrolyte, and a separator. The cathode is made up of a Li complex of transition metal oxide such as Lithium Cobalt Oxide (LiCoO2), Lithium Nickel Oxide (LiNiO2), Lithium Iron Phosphate (LiFePO4), etc.

During the first charge and discharge of a li-ion cell, anode material reacts with electrolyte, and after the reaction thin film is created on the surface of anode where Li+ ions can move forth and back, but electrons do not. The thin formed on the anode surface is called the solid–electrolyte interphase (SEI). Values for SEI thickness reported in literature tend to vary between roughly 1.5 and 584 nm, and it is mainly composed of various inorganic components  Lithium Fluorine (LiF), Lithium Carbonate (Li2CO3), Lithium Methyl Carbonate (C2H3LiO3), Li2O (Lithium Oxide), Lithium Hydroxide (LiOH), as well as some other organic components.

During a charging process, electrons are extracted from the cathode and transferred to the anode via the external circuit. At the same time, Li+ ions are de-intercalated from the cathode and transported to the anode via the electrolyte. The whole process is reversed during discharge.

During the charging of lithium-ion batteries, there are three main steps in the intercalation process:

• diffusion of Li+ ions out of the cathode;
• diffusion of solvated Li+ ions in the electrolyte;
• de-solvation of solvated Li+ ions in the electrolyte, and the intercalation of ions into the graphite interlayer through the SEI.

Challenges of Lithium Plating

The visible consequences of li-ion battery degradation are capacity fade and power fade. Capacity fade means the usable capacity of the cell decreases, while power fade means the deliverable power of the cell decreases after degradation.

Lithium plating in lithium-ion batteries (LIB) has been recognized for a considerable time as harming both battery longevity and safety.

Lithium plating is metallic lithium deposits on the anode surface that have not inserted themselves into the anode material via intercalation. Lithium plating can be caused by a combination of factors such as low temperatures, high state of charge (SoC), high charge current (kinetic plating), high cell voltage, and insufficient anode mass or electrochemically active surface area.

Even at moderate charge rates, below-freezing temperatures slow down the main intercalation reaction enough to cause plating.

During charging at sub-zero low temperatures, ionic and electronic conductivity of electrolytes and the internal reaction rate of the battery decreases. With a high charge rate, a substantial quantity of lithium ions migrates towards the negative surface. Moreover, when the concentration of lithium ions saturates at the solid-liquid interface, lithium ions will also accumulate on the surface of the SEI film resulting in the formation of lithium plating on the SEI film's surface. The capacity degradation caused by this portion of the lithium plating can be restored, but the unrestored lithium plating continues to be in the growth of the negative SEI film. If the irreversible part of the lithium plating mostly keeps accumulating on the surface resulting in the rapid thickening of the SEI film it can lead to the formation of lithium dendrites. Some dendrites of lithium can fall off and form “dead lithium”, causing a greater loss of active (cyclable) lithium. Lithium that is “dead” no longer participates in energy transfer, resulting in a decrease in battery capacity.

However, certain lithium dendrites present on the plating may continue to grow and potentially puncture the separator when subjected to mechanical stress. This can lead to a short circuit and ultimately result in battery failure, posing a fire hazard!

Thermal Management and Preheating Strategies

The primary objective of battery thermal management at low temperatures is to recover the energy and power capabilities of Li-ion batteries, while also eliminating lithium plating. This can be achieved by heating the batteries from subzero temperatures before their operation.

In cold climates, the preheating process in electric vehicles (EVs) usually results in additional energy usage. As a result, heating batteries at low temperatures becomes more difficult compared to cooling them at higher temperatures.

To maintain cabin comfort, preserve battery energy, and extend the operating temperature range of batteries, it is essential to optimize energy usage in preheating systems.

The design factors of the preheating system are as follows:

• the amount of electrical energy used;
• the time it takes to preheat;
• the impact of heating on battery aging;
• the consistency of temperature across the battery cell, module, and pack;
• the total cost, which includes system cost, operational cost, and maintenance cost;
• the added complexity, such as extra devices, weight, and space required for the heating system integration;
• the safety and reliability of the heating system.

What do you think about these issues? Let's discuss the real situation and potential advancements in this field. Share your thoughts and join the conversation in the comments!