The minimum requirements on the life of an LIB for the storage of solar energy are a cycle life goal of 30,000 cycles and a 15-year calendar lifetime.

Although lithium-ion batteries (LIBs) have been in use since the early 1990s in computers, cell phones and cameras, their application as a storage device for large-scale solar requires further technological development.

Specifically, measures need to be taken in order to lower the cost and improve the safety of using LIBs as a solar storage medium, as well as increasing their ability to withstand abuse. Utilising materials alternative to those used in standard LIBs, designing appropriately–sized battery packs and deploying the most suitable battery management system can address the cost and safety issues associated with LIBs.

What is an LIB?

An LIB consists of a cathode (positive) and an anode (negative) on either side of a porous separator that is soaked in the electrolyte. The electrolyte normally consists of a lithium salt dissolved in a mixture of organic solvents. Energy is stored and released in an LIB as lithium ions shuttle between the cathode and the anode.

Cost issues associated with LIBs

Materials

The active materials in LIBs constitute the main cost. Cathode materials are the most expensive materials, accounting for 30–40 per cent of the total raw material cost, while the anode and electrolytes account for approximately 20 per cent each. The costs of cell assembly, packaging and manufacturing represent approximately another 20 per cent.

Cycle life and calendar life

The cost per cycle of a battery is a practical way to compare the financial efficacy of different batteries or battery systems. Cycle life is the number of cycles that a battery can perform before failure. Calendar life is the length of time a battery can undergo some defined operation before failure. The minimum requirements on the life of an LIB for the storage of solar energy are a cycle life goal of 30,000 cycles and a 15-year calendar lifetime. The currently commercial available LIBs can only achieve 1000-2000 cycles with a 3-5 year calendar lifetime.

Safety issues associated with LIBs

Currently used LIBs have aroused safety concerns due to the use of:

• Highly reactive materials
• Highly flammable organic electrolyte
• Poor ability to withstand abuse
• Improving cost and safety issues with LIBs.

Selecting alternative materials for cathodes and anodes

The materials that can be used as cathodes and anodes in LIBs to store solar energy are limited. The most promising cathode and anode materials are lithium iron phosphate and lithium titanium oxide. By coupling these two materials, a cell potential of approximately 1.9 V can be achieved. Although this is lower than the standard LIB 3.5 V, this combination of materials provides a high degree of safety, long cycle life, and rapid charge capability. The precursor materials for manufacturing both materials are cheap, but the cost for synthesis of lithium-ion phosphate is still high. Large-scale preparation can be a better solution to lower the processing costs.

Selecting alternative materials for electrolyte

The currently-used organic electrolyte is based on different types of carbonate, which are highly flammable. As an alternative, ionic liquids have good thermal stability, but the cost is high. Polymer and solid-state electrolytes would be better choices for next generation LIBs with improved safety.

Improving cell packaging

Particular power and energy requirements can be achieved by packing together different numbers of battery cells. In order to achieve optimal output and function, the cells should have identical performance rates, identical internal resilience and identical voltage and capacity.

It also needs to be considered that the bigger the cell pack, the more dangerous it is: owing to the large amount of materials in use, which are very difficult to control in the case of a system failure. From a productivity, safety and cost point of view, the best LIB cell to use in a packing scenario is the cylindrical ‘18650 cell’ (diameter: 18 mm, length: 65 mm).

Designing superior battery management systems

Battery management systems (BMS) monitor and control each battery in a cell pack. Not only does a BMS monitor and control each individual cell, but it also addresses safety concerns such as short circuits, overcharge/discharge, and overheating. A BMS also manages the package of cells if one of them has become dysfunctional. A BMS is expected to meet a lot of demands, and as such is an important area of research in the pursuit of improving the viability of LIB for solar storage use. Of most significance is the need for next generation BMS to be able to manage safely and efficiently, a large number of LIB cells in a pack.

Implementing improvements

In summary, close collaborations between researchers and industry partners from different areas, including materials engineering, informatics, electronic engineering and chemistry, are needed in order to overcome all the challenges mentioned in this article. Nonetheless, it is my opinion that using LIBs as a large-scale solar energy storage system is not too far from being a reality.