Li-ion Cell: The Inner Working
Li-ion cells make use of an intercalation process where Lithium ions (Li+) move like atomic “ping-pong” balls between the anode (-) and cathode (+) active material inside the cell during discharge and charge. The active material has a crystal structure that enables the Li+ to squeeze into the atomic cavities (intercalation). The material is applied as a very thin coating on aluminium (-) or copper (+) film, which then acts as the current collector.
Li-ion cells are not the same as Lithium cells. The latter have a metallic lithium anode and is not rechargeable.
This allows the electron, which has been stripped off the Li atom to create the Li+ ion, to travel via wires outside the cell to the other terminal where the Li+ have intercalated into. One can harness the energy which the electrons carry in this process, and that is how a battery can power a light or a motor. A charged cell has a fully lithiated anode, and the cathode is almost empty.
A single cell will have a nominal terminal voltage [V] and will be able to produce a certain current for some duration, measured in Amp-hours [Ah]. The energy that a cell can deliver is the Voltage multiplied by its capacity in Amp-hours and is measured in Watt-hours [Wh].
There are various types of active materials that can be coated onto either the anode or cathode, and depending upon the combination, a Li-ion cell can have substantially different voltage, power, and safety characteristics. Most cells make use of Graphite on the anode. The most common chemistry for the cathode found in Laptop cells is Lithium Cobalt Oxide (LCO), giving 3.7V. It has a very high energy density but was the main culprit in the early days of battery fires.
Safer chemistry is Lithium Iron Phosphate (LFP, or sometimes called LiFe), but its voltage is lower at 3.2V, resulting in less energy per mass. Full electric vehicle batteries need to be safe and light, so manufacturers are using Nickel Manganese Cobalt (NMC) and even a mixture of other chemistries.
One of the main limitations for high power (large current) applications is the physical damage caused by the ions as they rush in and out of the Graphite on the anode. Lithium Titanate Oxide (LTO) does not experience any damage, and that makes it a good anode material for very high power applications.
The type of cell and management system in Balancell batteries ensure that we can guarantee a cycle life of more than 5 years with unlimited cycles or with a battery life-time energy delivered measure in kWh.
The drawback is that the cell voltage is only 2.4V, so much less energy per mass. Some high-end cordless power tools make use of LTO cells. As with the anode, the cathode material also “wear out” with use, resulting in a loss of capacity. This is a function of charge and discharge rate, as well as chemistry. This kind of ageing is called Cycle Life.
The electrolyte inside the cell allows the Li+ ions to “swim” from one electrode to the other side. It is usually a strong solvent, containing LiPF6 salt dissolved inside it. This solvent attacks the active material’s exposed surface and causes a Solid Electrolyte Interface (SEI). One can imagine it as a kind of protective “corrosion layer” like what one sees on aluminium.
Unfortunately, it also greatly hampers the Li+ movement, which causes the cell’s internal resistance to increase. The SEI layer grows with time, irrespective of use, and accelerates exponentially with temperature. This is called Calendar Life. The increase in internal resistance is the biggest reason for phone and laptop batteries to seem to lose capacity.
It’s not that the cell has reduced in size, it’s just that the internal resistance has increased such that the voltage drops to a point where the battery management system (BMS) thinks the cell is empty. In most cases, one can get the same amount of amp-hours out of the cell if you just discharge it much slower (that’s why battery saving mode on cell phones can extend the on-time so dramatically – it’s not just a slower drain in energy).
Since the electrolyte is an aggressive solvent, it is often quite flammable. Some cell manufacturers add fire retardant chemicals to the electrolyte, which makes the cell “safer”. In the early days of cell safety research, people tried to make use of a solid polymer electrolyte, instead of a liquid solvent. This was called a LiPo (lithium polymer) cell. To get it in-between the electrodes, these cells were manufactured in flat layers and packaged inside a plastic-covered aluminium pouch.
Polymer electrolytes were not very successful, but the pouch-type cell turned out to be a very versatile packaging. Nowadays one gets a large range of pouch cells, all with different chemistries and sizes, but they mainly contain liquid electrolyte inside. The name LiPo tended to stick, and people would very often call pouch cells LiPo cells, which is kind of a misnomer and does not say anything of the cell’s chemistry or other characteristics
Battery State of Charge
The actual resting voltage of a Li-ion cell can differ substantially between full and empty. The shape and absolute voltage are dependent upon the cell chemistry. The state of charge (SoC) or inversely the depth of discharge (DoD) of a cell with a moderately sloping curve can be estimated by simply measuring the terminal voltage when it’s resting (i.e. no current flow for some time). This is known as the Open Circuit Voltage (OCV), and is moderately dependent upon temperature, but remains virtually unaffected by age.
If there is a small and fairly stable discharge current, then one can also make a reasonable estimate of the SoC by looking at the present terminal voltage. This is the simplest way and is often employed in low-cost consumer devices that use LCO and NMC cells. One cannot successfully use voltage as an SoC indication for LFP cells because of the very flat portion in the middle. It is also a very poor indicator for motive applications where the discharge current can be quite high and fluctuating during acceleration.
A more accurate method of SoC prediction is utilizing Coulomb Counting, or in layman’s terms, measuring the Amp-hours discharged by the cell. Coulomb counting involves measuring the current in time increments, and then sum the product of current and time step, which gives you Coulombs [A∙s, or Ah].
Unless a cell has physical damage inside (like excessive fast charging), the true capacity of a cell in Amp-hours (or coulombs) remains the same, and one can subtract the “counted” discharge coulombs from the true capacity, to know the coulombs that are left in the cell, assuming you know you have started with a fully charged cell.
It is not practical to extract all the coulombs from a cell, hence battery designers will often report 0% SoC (empty) when there are 1-5% coulombs still left in the cell. This also helps to prolong the cell’s life, as operating in the very low voltage range can accelerate the cycle and calendar life.
The biggest drawback with coulomb counting is that one needs to measure current very accurately in small time steps to get a good Ah count. If one misses a short current spike or a prolonged small current, the coulomb counting can drift, and one would need to “reset” it at a known point. The only known point is at 100% fully charged, but if your system makes use of regular interrupted opportunity charging, the time to reset may come very late.
Balancell batteries’ state of charge is predicted by a very accurate coulomb counting method. The system continuously measures currents as low as 1mA, 3000 times a second. This means there is less than a 0.1% drift per day on the actual remaining Ah. The resolution and accuracy are comparable to lab-scale measurement equipment. This is all due to a dedicated precision-engineered Batter Energy Meter (BEM).
Battery Management System
The BMS of a Balancell battery consists of CMMs connected to each cell, a BEM that contains the “brain”, and a Protector that manages the live state of the battery terminals. The BEM is in regular communication with the back-end to wirelessly communicate all the important battery information. It also has an externally mountable State of Charge display for the user.
Because of the safety concerns of Li-ion batteries, they all should have some kind of Battery Management System (BMS). The BMS can perform a large range of functions, but they would typically do the following:
- Monitoring of pack and/or cell voltages, temperature and currents
- Protection against over-charge, over-discharge, over-temperature, over-current, under-voltage
- Managing of charge acceptance, cell balance, pre-charging
- Reporting of state of charge, state of health, use profile, etc.