In this article we focus on Lithium batteries and their application
BESS systems can use a variety of battery types with relative advantages and disadvantages that are worth considering. For example, Lithium Iron Phosphate (LFP) batteries offer longer term deep cycle durability than Lithium polymer (LiPo) and they are resistant to dendrite growth so they pose no fire risk. Their day-one capacity is a little lower than LiPo, but after a few hundred cycles they'll hold up better in capacity.
Nickel-Manganese-Cobalt (NMC) batteries, on the other hand, have a shorter deep cycle life expectancy than LFPs but they offer increased power density and considerably better cold weather performance, particularly in charging, which can reduce operating overheads. It's for these reasons that NMC and LFP batteries are increasingly prevalent in BESS applications.
Lithium batteries function through electrochemical reactions involving lithium ions moving between the battery's positive (anode) and negative (cathode) electrodes, with material motion blocked by a separator that allows ion transport in the electrolyte. Lithium batteries typically contain a cathode (the +ve) formed from a lithium compound such as LiCoO2, NCA, NMC, LiFePO4 and LTO. These remain typical in the newer, solid-state embodiments.
An anode (the -ve) is usually made of carbon (graphite or graphene). Coatings applied to the electrodes aid as barriers to the formation of dendrites, metallic threads that form on the surface of the electrodes and can pierce the separator and cause short circuits. These coatings include polymers or ceramics, depending on the manufacturer. Between the electrodes there is typically:
- Electrolytes: There are three classes of electrolytes used in lithium battery technology including:
- Liquid electrolyte: A lithium salt dissolved in an organic solvent, often containing flame-retardant additives. The lithium salt is the ionic conductor that transfers charge; the organic solvent delivers high ionic motility and the additives optimize the stability, conductivity, and safety of the electrolyte.
- Polymer-based gel electrolytes: They deliver high ionic conductivity but a much-reduced chance of leakage. The polymer matrix acts as a gelling agent and, by design, they don’t pose a barrier to ionic motility within the solvent. The lithium salt acts in the same way, but gel-type cells offer improved battery safety and cycle life.
- Solid-state electrolytes: New alternatives to liquid electrolytes that improve safety and stability, acting as a barrier against dendrite formation and improving the thermal and chemical stability of the battery. This facilitates higher charge and discharge rates without increased risk.
- Separators: Porous membrane structures that force a physical gap between the anode and cathode, while allowing lithium ions to pass through during charge and discharge. Separators are generally constructed from high-porosity polyethylene (PE), often containing a polypropylene (PP) element to improve robustness.
During charging, lithium ions are electrically “pushed” from the positive electrode to the negative electrode through the electrolyte and become adsorbed onto/into the anode carbon. Electrons flow from anode to cathode in the outer circuit during charging. The charge current pushes electrons from anode to cathode. During discharge, these ions move back to the positive electrode, releasing electrical energy, and current flows in the outside circuit from cathode to anode. This ionic movement is greatly assisted by the crystalline structures within the electrode materials and energized by the flow of electrons through the external circuit in both charge and discharge.
The voltage and capacity of lithium batteries vary with the electrode/electrolyte chemistry and internal design, with voltages ranging from 3.6 V to 3.7 V per cell. Capacity relates to the amount of electrolyte and the size and construction of the electrodes. The discharge rate depends on many details and the internal safety systems in the cell and the battery. These prevent overcharging, over- discharging, and thermal runaway. Source