Inside Modern Li-ion Batteries: Cell Chemistry
Li-ion batteries can hold substantial amounts of energy in a very small package. Lithium by itself is extremely reactive with most other elements, and the carbon/graphite inside the cells, together with flammable electrolytes make Li-ion cells a potential fire hazard. The cells also typically have a very low internal resistance, enabling them to produce extremely high and damaging currents when short-circuited. In the early days, Li-ion safety was poorly documented and Li-ion cells would self-ignite, resulting in reputation-damaging recalls. Many of these incidents could be linked to internal defects due to poor quality control, contamination or degradation. Most of these problems have been eliminated through the natural maturity of the cell manufacturing industry.
Cell safety incidents currently can mostly be attributed to abuse. Cell manufacturers and battery designers add additional features to their products to make them more abuse tolerant:
External Short Circuit
Some cells contain an internal “fuse”, which would melt and thus disconnect the cell internally in case of a hard internal short circuit. In large batteries, there would always be a fuse in the string of series-connected cells. There may even be a current monitoring circuit that will disconnect the battery much faster than what the fuse can respond to.
The cells used in Balancell batteries do not have an internal fuse, but the battery is equipped with either a 200/300/400A fuse. The Smart-E Switch will also disconnect the battery safely at currents exceeding 550 – 1000A, which means there is no risk of damage during battery installation. The batteries also have a dedicated on/off button to add another level of safety.
Balancell’s batteries all have an intelligent Smart-E Switch, which will prevent the cells from being overcharged, irrespective of the type of charger used. The cells all have burst disks which will rupture when the pressure increases substantially. Also, the cells have been tested and proven to withstand an overcharge of 25% without any catastrophic damage.
If you were to overcharge a flooded lead-acid cell, the excess energy is released in the form of hydrogen gas and/or water vapor.
Remember that the process of charging a cell “pushes” the Li ions from the cathode into the anode active material? When the anode is full, the cell cannot store any further energy added to it. The result is that the energy is converted into heat or other unwanted chemical reactions. This is why Li-ion cells should only be charged by a device that will prevent overcharge. Some cell manufacturers would add other chemicals in the electrolyte that would “burn-off” the excess energy when the cell is full – this is called shuttle chemicals. These can only do so much, so inevitably the heat generated will result in the gassing of the electrolyte. For pouch cells, they will typically balloon. Cells with hard casings will have a burst disk of some kind that will rupture at a certain pressure, thereby often releasing the electrolyte and disabling the cell from receiving any further charge.
The chemical reactivity of the electrolyte in Li-ion cells is voltage dependent. If the cell voltage gets below 2.0V, it will start to dissolve the copper in the anode electrode. The cell may still appear perfectly fine, and it is very possible to re-charge the cell and continue to use it without any noticeable change. However, the dissolved copper does not return back to the electrode, instead it will plate on the surface of the active material during charging. Over time, this plated copper can form a dendrite that can pierce the separator and result in an internal short circuit, which can cause self-ignition. Therefore, NEVER recharge a cell that has stayed below 2V for a few hours or longer. DISCARD the cell.
The BMS of Balancell batteries will shut down the battery and prevent any further discharge when any cell reaches its safe lower voltage. It will continue to monitor the battery and send status information to the backend. The monitoring service that Balancell offers their customers includes a warning if any cell does go below the 2V limit.
The most significant cause of early cell “death” is excessive temperature. This is because the SEI layer growth increases exponentially with temperature (see the Arrhenius equation). At above typical operating temperatures (25° – 50°C), the SEI layer and electrolyte can start to produce gasses, which would result in ballooning or pressure increase. At even higher temperatures (above 150°C), the active material can be decomposed into its constituents.
The Cell Management Module (CMM) that is connected to each cell in Balancell’s batteries monitors the cell temperature and reports this on regular basis to the back-end. The battery will enter a self-protection mode if any cell temperature exceeds -5°C or 55°C.
If one has a Lithium Cobalt Oxide material, for example, the active material becomes a self-oxygenated fuel, and there is no way to stop the chemical reaction that follows. This is normally called the cell thermal run-away limit. The LFP chemistry (LiFePO4) “holds on” to the oxygen much better, and that is why LFP cells are much less prone to fire hazard. The temperature limit for NMC cells is also higher than for LCO, hence its popularity in electric vehicle applications. For a good paper on the science around cell thermal runaway, see the paper by Feng et. al.
Balancell’s BMS monitors each individual cell voltage and will ensure that all cells are balanced at the end of charge by activating passive balancing resistors on the cells with a higher voltage than the rest. This is done by the CMM connected to each cell. The BMS also have an undervoltage cut-out protection which is based on individual cell voltages.
For batteries that are made of a series of cells, there is the risk that a weak cell can be forced into reverse polarity. This occurs during discharge when one cell reaches its maximum Ah it can discharge (i.e. it is fully discharged), but the surrounding cells can still produce current. The current is then “forced” through the empty cell, thereby forcing its polarity to reverse. The process also actively erode the copper electrode and can result in copper dendrites forming on the cathode causing an internal short circuit. This is very damaging to a cell and will result in substantial heat production. This situation can happen if all the cells in the series string are not of the same capacity, or if one of the cells is at a much lower state of charge than the others. This is termed an unbalanced battery.
No battery is fire-proof, but some will be able to withstand higher levels of physical abuse than others. Most commercial cells need to pass some abuse tests, which include piercing it with a nail, dropping it from a certain height, squashing it on the side using a blunt object, and sometimes even dropping it into salty water. In all cases, the expectation is that the cell does not explode or catch fire, although they often release plumes of white smoke and can eject hot slivers of melted plastic and copper electrode shards.
The cells Balancell uses all pass the nail puncture, crush and drop test. Additionally, the cells are packaged in a sturdy 3mm aluminium box with vertical cell restraints. This means you can even tip the battery upside down without any risk. The battery box is sealed to an IP63 rating, allowing one to even clean it with a high-pressure water jet without the risk of water damage.