Lithium ion batteries are ubiquitous in portable electronics and electric vehicles, but are now cost-competitive in stationary storage applications where lead-acid and nickel-metal hydride technologies once dominated.
“Lithium-ion” may refer to a number of technologies which use an electrolyte composed of a lithium-salt dissolved in an organic solvent. A graphite (carbon) anode is typically used, though alternative anode technologies are being widely investigated.
The name of the technology generally relates to the cathode in use. As an example, a lithium cobalt oxide (LCO) battery has a carbon (C6) anode and a lithium cobalt oxide (LiCoO2) cathode. Li+ ions move between the two during charging and discharging, as electrodes swell and contract to accept/give-up ions.
If energy density is thought of as the amount of energy stored per unit of weight/volume, and power density is the maximum power delivery per unit of weight/volume, then the following can be said of each lithium-ion chemistry:
- Lithium Cobalt Oxide (LCO): commonly used in portable electronics as they offer the highest energy density of commercial lithium battery technologies. The use of cobalt results in high energy density, but it is an expensive metal that displays thermal instability (unsafe) and fast capacity fade (short life) as a cathode material. These characteristics, combined with low power density, result in other lithium-ion chemistries being preferred for EV and stationary storage applications.
- Lithium Manganese Oxide (LMO): once the chemistry of choice for EV manufacturers, the use of manganese in place of cobalt allows for higher power density and greater thermal stability when compared to LCO. However, lifetime remains short, and energy density is lower.
- Lithium Nickel Manganese Cobalt Oxide (NMC): blending nickel with manganese and cobalt oxide improves cathode lifespan. Combining all three results in good performance across all metrics discussed thus far (energy density, power density, lifespan, and thermal stability). NMC can be thought of as an “all-rounder” chemistry, and production volumes are increasing owing to their suitability across many applications, particularly EV’s.
- Lithium Nickel Cobalt Aluminium Oxide (NCA): competes with NMC for market share in EV powertrains. They exhibit high power density, high energy density, and long shelf life, but degrade more quickly with use than NMC cells. The increased cobalt content improves energy and power density, but also makes cells more expensive and less thermally stable.
- Lithium Iron Phosphate (LFP): high thermal stability, long lifespan, and cheap cathode materials make LFP the obvious choice for stationary storage applications. However, owing to low energy density they are unsuited to EV’s, and manufacturing volumes have not yet reached the point where system costs reflect the low materials costs.
- Lithium Titanate (LTO): with LTO nanocrystals used as the anode, these batteries display unparalleled thermal stability and lifespan. They are expensive when assessed on the basis of energy storage capacity ($/kWh), but their ability to deliver this energy over an extremely short period makes them competitive in high power applications. With low energy density making them unsuited to EV’s, expected applications include grid frequency regulation and PV/wind farm smoothing.
For stationary storage applications such as on-grid and off-grid solar storage, LFP and NMC chemistries are the market leaders from the lithium-ion family. The majority of market share remains with incumbent flooded and sealed lead-acid batteries however, and the following parameters should be considered when comparing the two technologies.
Lithium-ion batteries should not be over-discharged. If the cathode cannot accept any further lithium ions, metallic lithium plating may occur resulting in irreversible capacity loss. Further, If the voltage of the battery is allowed to fall below its minimum threshold, the copper electrode can dissolve into the electrolyte. When the battery is subsequently recharged, a short-circuit can form between electrodes. The BMS is responsible for protecting battery cells from this over-discharged state.
The rate at which the battery is discharged will affect the total capacity for both lead-acid and lithium-ion chemistries. Higher rates lead to higher inefficiency, due to heat loss, and higher cell temperatures that increase unwanted side reactions and accelerate capacity fade. Similarly, there will be a maximum discharge rate above which I2R losses (on account of the internal resistance of the battery) could lead to overheating and thermal runaway. Some BMS protect batteries against over-current conditions, though external fusing may also do this job.
In terms of performance, lithium-ion batteries outperform lead-acids significantly owing to their lower internal resistance. Less heat is generated during fast discharging, reducing side reactions and improving efficiency. However, there are additional safety concerns with lithium-ion batteries in under-voltage and/or over-current conditions and, as such, a BMS is required.
Within the lithium-ion family, there is a large variation in the ability to discharge quickly and safely. LTO performs best, showing extremely low internal resistance and remarkable long term tolerance to high discharge rates upwards of 4C (15 minute full discharge). At the other end of the spectrum is the sluggish LCO cell, which provides the highest energy density but cannot deliver this energy quickly.
Generally speaking, the low internal resistance of lithium-ion batteries allows them to discharge faster and more efficiently than lead-acid batteries. This ability to deliver high power makes them well suited to wind and solar farm smoothing, as well as for voltage and frequency regulation on electricity networks.
As above, high charge currents will cause batteries to lose energy to heat due to the internal resistance of battery cells. In addition to the decreased efficiency and increased potential for thermal runaway, the catalytic effect of heat on unwanted and irreversible side reactions accelerates capacity fade. Lithium-ion batteries are less sensitive than lead-acid’s in this respect, owing to lower internal resistances. Accordingly, they may be charged significantly faster than lead-acid batteries.
However, excessive charging currents can cause lithium plating of the anode, which is unable to accommodate (intercalate) lithium ions quickly enough. A similar situation occurs if the battery is over-charged – the anode, already full with lithium ions, cannot accept more ions, which instead deposit on the surface of the electrode. As well as increasing internal resistance and reducing capacity (due to active material loss), lithium plating can cause dendrite growth that forms a short circuit between electrodes. The BMS measures and balances individual cells to protect them from over-voltage conditions. Most also ensure cell temperatures do not exceed safe limits while some also protect against over-current.
While incomplete charging of lead-acid batteries will lead to capacity reduction via sulfation, lithium-ion batteries prefer incomplete charging as electrodes are least stressed when ions are shared between electrodes.
As with a lead-acid battery, a lithium-ion battery is charged via a constant current phase until the cell voltage peaks. This voltage is then maintained until the charge current falls to 3-10% of its constant current phase. Float, or trickle charging, is not applicable to lithium-ion batteries, as they prefer partial SOC operation. Instead, chargers will typically switch off until the battery self-discharges down to some setpoint SOC, before charging recommences.
This preference for a partial SOC has a significant impact on its suitability for diesel hybrid power systems as it allows for batteries to be left discharged while generators deploy, until such time as renewable generation is sufficient to serve loads once more. In this way, batteries are never charged by anything other than excess renewable generation. Deployment of generators in this manner is often referred to as a load-following dispatch strategy.
Where batteries cannot be left discharged, as in the case of lead-acids, generators are most often deployed to serve the load and charge the batteries. This method of generator deployment is most commonly referred to as a cycle-charging.
For a given battery bank capacity, a load following strategy will result in a higher overall renewable energy contribution than for a cycle-charging strategy and, conversely, for a given renewable energy contribution, a load-following strategy will necessitate a smaller battery bank than a cycle charging strategy. Due to the high capital cost of storage, the generator deployment strategy has a significant impact on the levelised cost of energy of diesel hybrid systems. Accordingly, this should be considered in any comparison of the costs of battery technologies.
Electrical (Coulometric) Efficiency
The ratio of the energy required to charge a battery compared to the available energy during discharge is referred to as the efficiency. A typical lithium ion battery will lose only 5% of energy round-trip (95% efficiency), compared to 20-25% losses for lead-acid systems.
Both lead-acid and lithium-ion technologies perform well with regards to self-discharge, with losses of around 5% of capacity per month. In frequent cycling applications this loss is of little consequence.
Though lead-acid batteries consist of highly toxic lead and corrosive sulphuric acid, they are generally considered safer than lithium-ion batteries owing to a lower risk of thermal runaway.
Thermal runaway refers to the positive feedback loop that can cause battery swelling, fire and/or explosion when the catalytic effect of heat released during battery malfunction accelerates the irregular reactions causing the release of heat.
Due to the higher energy density of lithium-ion cells, the reactivity of lithium, and the flammability of their organic solvent electrolyte, thermal runaway can be more dangerous than in lead-acid battery cells. Without protection systems in place, the likelihood also tends to be greater (with thermally-stable LTO batteries the exception).
In practice, lithium-ion batteries are manufactured with a number of safety measures to offset this risk, and typically include:
- Battery management systems (BMS) that isolate battery packs in the event of over/under-voltage or over/under-temperature conditions
- Cell balancing systems that equalise the SOC of battery cells connected in series, to avoid over/under-voltage conditions
- Battery fusing to arrest short-circuit currents
- Thermal management systems to carry heat away from cells via air or liquid cooling
The widespread use of lithium-ion batteries in EV’s is indicative of the effectiveness of these protection systems. In stationary applications, where temperatures are lower and more stable, the likelihood of thermal runaway is reduced even further.
Lifetime & Depth-of-Discharge
In ‘float’ mode, lead-acid batteries display long lifetimes up to 20 years. When cycled according to manufacturers’ guidelines they will tend to degrade linearly until power delivery and energy storage capacity have fallen to ~80% of their initial specifications (an 80% state-of-health). At this point, the likelihood of imminent failure is high and, for this reason, this point is taken as the end-of-life for lead-acid battery systems.
Manufacturers’ claims vary, but top-end deep-cycle lead-acid battery datasheets state cycle lives of 3000+ cycles for discharging down to a 40% state of charge (SOC). Regularly discharging beyond this point will hasten degradation of the battery and, conversely, more shallow discharging will increase the cycle life. The overall energy throughput is the same in either case, and this largely determines battery aging, alongside charge/discharge rates and ambient temperatures.
With proper operation lithium-ion batteries also degrade linearly with time and use, but do not display this tendency to fail at ~80% SOH. Instead they will continue to lose capacity until the battery is no longer practically useful. This characteristic has opened up battery re-use schemes, particularly for degraded EV batteries that can be refurbished for stationary storage applications, where power/energy density are no longer as important.
Top-end lithium-ion manufacturers now warrant batteries for 10 years down to a 60% SOH. Expected cycle lives (before this point is reached) are frequently claimed to exceed 4000 cycles, though large differences are seen between manufacturers’ claims and cell chemistries.
If manufacturers’ claims are accurate, the superior cycle life of lithium ion batteries (excluding LCO) would make the additional capital expense (per kWh) worthwhile when the battery is to be frequently cycled. The long shelf life of lead acid batteries would continue to make them suited to standby applications such as UPS or backup power systems.
The Lithium-ion Battery Trial aims to determine the accuracy of these claims when batteries are subject to ambient temperatures typical of the Australian climate.
Energy & Power Density
Where space restraints exist, lithium ion batteries are often preferred to lead-acids owing to a high energy and power density in terms of both size and weight. This energy density is reflected in the widespread deployment of lithium ion batteries in portable electronics and electric vehicles.
Lithium-ion battery packs are already on the market that provide energy densities of 144 Wh/L and 107 Wh/kg (compared to 80 Wh/L and 34 Wh/kg for lead-acid).
Toxicity & Disposal
Though primary (non-rechargeable) lithium batteries possess toxic metallic lithium, the components of secondary (rechargeable) lithium-ion batteries are much more stable. With recycling initiatives in their infancy, lithium ion batteries are most often disposed of in traditional waste streams.
As large-format lithium-ion battery sales accelerate due to the expanding EV and stationary storage markets, these recycling options will necessarily expand. At present, disposal and recycling options for lead-acid batteries are much more advanced.
There are many factors to consider when comparing the cost of lead-acid and lithium-ion batteries. A simple comparison of the $/kWh is not sufficient to make the best decision, not least because the kWh stored in a battery depends on the rate at which you intend to charge/discharge it, or that this kWh capacity will fade at different rates for different batteries. The overall efficiency must also be factored in, and then the value of partial SOC operation must be considered.
What can be said is that, at low discharge rates (24 – 120 hours) lead-acid batteries will tend to have a lower capital costs per kWh of storage. However, this capacity will fade faster than lithium-ion batteries with frequent use (daily cycling etc.), and the system will have to be designed so as to ensure batteries are fully charged regularly.
As the required charge/discharge rate increases, the capital costs of lead-acid batteries (on a $/kWh basis) increase, and the economics begin to favour lithium-ion chemistries.
A similar situation occurs when the battery is to be frequently used. The slower capacity fade and the higher efficiency of lithium-ion cells does not impact the capital cost (on a $/kWh basis), but the levelised cost (or total cost of ownership) will tend to favour lithium-ion batteries.
An assessment of the lifetime cost of ownership must take these factors into account, alongside project constraints such as space and weight limits, serviceability, power access etc.
The most significant obstacles to lithium-ion deployment in stationary storage applications are capital costs and maturity. The lead-acid battery has been in use for over 100 years, deployed across many applications, and is as predictable and understood as is possible for an electrochemical technology. The same cannot be said of lithium-ion batteries where example projects of many years’ duration do not and cannot exist on account of the infancy of the technology.
While lead-acid technology is mature and prices are unlikely to fall significantly in future, lithium-ion prices are falling rapidly due to ongoing demand from the EV sector propagating technological improvements and streamlining of manufacturing processes.
Currently, manufacturers’ data sheets provide most of the available information, which is derived only from laboratory testing. ITP’s Lithium-Ion Battery Trial is one attempt to scrutinise manufacturers’ claims and understand how this new technology will perform in real-world conditions.