Lithium ion battery primer

Here is a simple diagram of a lithium ion (Li+) battery. The battery has three general components : anode, cathode and electrolyte.

The electrolyte: 

– separates the anode and the cathode to prevent short circuit

– contains a lithium salt (Li+ PF6- usually) that is dissolved in a solvent or matrix

– the solvent/matrix is typically an organic liquid but can also be a gel or an inorganic solid

– allows flow of lithium ions through it

– does not allow the flow of electrons (e-)

– creates a chemical interface at the border of both cathode and anode that is (a) highly complex (b) poorly understood and ( c ) believed to be the major cause of battery failure and/or performance degradation over time

The cathode:

– is the positive terminal

– has lithium in it when the battery is in a discharged state

– has no (or very little) lithium in it when the battery is in a charged state

– the cathode of your laptop and smartphone is likely lithium cobalt oxide

The anode:

– is the negative terminal

– has no (or very little) lithium in it when the battery is in a discharged state

– has lithium in it when the battery is in a charged state

– the cathode of your laptop and smartphone is likely graphite (yes, pencil lead)

Charging:

– causes lithium ions to diffuse from cathode to anode

– causes electrons to flow through an external electrical circuit with electrons following the same direction as the lithium ions to maintain charge neutrality

– potential energy is built up (do work by pushing the rock up the hill) by applying a voltage to the cell, i.e., plugging your phone into the wall

Discharging:

– causes lithium ions to diffuse from anode to cathode

– causes electrons to flow through an external electrical circuit with electrons following the same direction as the lithium ions to maintain charge neutrality

– built up potential energy is spent (let the rock roll down the hill to do work for you) powering your device

Other things:

– lithium diffusion (movement) from anode to electrolyte to cathode and vice versa is incredibly slow : 0.0000000000000015 cm2/s

– how fast lithium diffuses in and out of the cathode and anode determines how much power your battery can provide and how quickly it can be recharged

– how many lithium ions the cathode and anode can contain determines how long your battery will last, i.e., its capacity

– a balance of power and capacity is desirable for most applications with niche applications requiring extremes of either power or capacity

– chemical reactions that happen at the interface between cathode-electrolyte and anode-electrolyte can cause lithium ions to get stuck at those interfaces and not allow them to diffuse back and forth and thus a loss of battery capacity (and sometimes power) is observed

– we are using the same cathode and anode chemistry that we did 10 years ago for almost all portable device applications

– engineering and packaging has improved – chemistry only barely

– Moore’s law does not apply to rechargeable batteries: miniaturization does not appear to help battery performance – in fact it often hinders

– what matters is that very few new viable anodes and cathodes with appropriate characteristics have been developed to meet the tight demands of industry

– industry demands are: safety, operating temperature range, weight of the battery, volume of the battery, power output, capacity, cost of materials, ease of processing, scalability, longevity (how many charge/discharge cycles) and durability

– 99% of claims that are hyped by PR pieces in popular science media regarding “groundbreaking” discoveries about new batteries fail horribly in several of the above industry requirements – most often in scalability and longevity

I know, these last few items seem very depressing but don’t fret. I’m confident that some of the problems will be solved regarding a next generation of lithium ion batteries. Also, the “beyond lithium” movement is well on it’s way with regards to charge storage and there are a lot of good people working on interesting problems. In a future post I’ll talk about what I think the ultimate fate of rechargeable batteries will be. Hint: I don’t think it’s necessarily electric cars.