Battery life is an explosive issue. Literally, as Samsung is discovering to its dismay. The company’s Galaxy Note 7 smartphone was praised upon release for best-in-class battery life, far outpacing its key competitor, the iPhones 6S and 7 Plus. Then it started blowing up. Samsung issued a recall and replace programme, and the replacements also started blowing up, forcing the company to suspend production entirely.
The affair marks the latest road block on the long fight to improve the batteries that power our electronics. While processing speed doubles around every 18 months, battery capacity takes almost a decade to improve to the same degree. That gap is starting to cause problems, but as Samsung has found to its cost, it’s not easy to fix.
A smartphone often lasts less than a day, a laptop sometimes only a few hours and an electric car struggles to go 350 miles. So why is it that battery life is still such a problem – and when are we going to fix it?
What is a battery?
Batteries are small containers of chemical energy. When a smartphone is plugged into the mains, electricity is used to reset a chemical reaction within the battery, transferring electrons from the negative anode to the cathode – the positive end of the battery.
Once charged, the battery can then create electricity by driving electrons through a circuit, in this case a smartphone, to the anode and will continue to do so until all of the electrons contained within the battery have transferred to the anode or a built-in switch disconnects the battery.
What is a battery made of?
Inside a typical battery you have an anode, a cathode and electrolyte – something for the positive ions to travel through.
Lithium-ion batteries found in most smartphones and electronics have a metal oxide cathode made of a cobalt, nickel, manganese or iron mix, a porous graphite anode that holds lithium ions within it and a lithium salt electrolyte.
Positively charged lithium ions travel through the electrolyte from the anode to the cathode driving electrons through the smartphone as required and back to the anode.
Why doesn’t it last long enough?
The principle of the battery may be simple, but the chemistry and technology to make it work is not. The major limiting factor for batteries is their energy density.
A battery can only generate as much electricity as its chemical components can store energy. Everything that is not the active material within the battery is effectively dead weight, including the casing, the controller chips, the wires to carry the current out – they all add weight but not power.
A typical lithium ion battery within a smartphone has an energy density around 150 Watt-hours per kilogram (Wh/kg). While Lithium ion battery energy density has improved since its introduction in the early 1990s, it is held back by its construction and chemistry.
The only way to immediately increase a smartphone’s battery life with current technology is to increase the power efficiency of the smartphone’s electronics and increase the size of the battery – but thinner and thinner smartphones demand thinner and thinner batteries.
Why does battery life diminish?
Battery life doesn’t stay constant for the entire life of a smartphone – it diminishes slowly over time, as the battery is discharged and recharged.
This is because the chemical reaction that produces the electricity causes thin layers of lithium to be laid down on the electrodes, which reduces the amount available to generate electricity and increases the internal resistance of the battery.
The higher the resistance the harder the battery has to work to maintain a usable voltage and so the amount of power it can produce per charge decreases. You might remember this bit from school:
Voltage = Current x Resistance (V=IR)
Why do some batteries explode?
Batteries with much higher energy density than lithium-based cells are already available, but they aren’t safe enough for use in portable electronics.
“The more energy you put into a box, the more dangerous it’s going to be,” says Dr Billy Wu, lecturer at Imperial College London’s Dyson School of Design Engineering. “Safety is absolutely key and thermal management is crucial. If a battery heats up beyond 80C you hit what is called thermal runaway, where the components start to decompose, and that’s when it can explode.”
The specific cause of Samsung’s issues with exploding batteries is unknown, the company just cites “a battery cell issue”.
What happens next?
In the immediate term, battery advances will come by bringing existing lithium-ion technologies closer to their theoretical limits, which will increase the power density of batteries.
A typical lithium-ion battery using lithium manganese oxide has a theoretical power density of 280 Wh/kg, but the final product only has 150Wh/kg so there is certainly room for improvement.
“It’s about optimising the structure within the battery,” says Wu. “If you imagine inside your battery you have this porous structure full of the active material.”
“For higher power output, you need a more porous structure to increase the surface area and allow more lithium ions through at any one time, but because it’s got more holes it holds less active material, which in turn gives you lower capacity.”
New, advanced battery chemistries such as lithium-sulphur and lithium-silicon are also being worked on, with companies around the UK currently developing the technology.
What is the future of battery technology?
Solid state batteries are one possible future, where the liquid electrolyte in the battery is replaced by a solid substance, which will provide significant safety improvements.
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