All energy is difficult to store, not just eletrical. Indeed, electrical energy is quite easy to store once you consider the big picture.
If you look at a tank of gasoline, you can see "wow, what a great storage for energy!". But while gasoline is great once you have it, consider how it was created in the first place:
- Fusion reactions inside of the Sun liberated some of the potential energy of a couple of protons
- The released energy propagated throughout the Sun (this already takes a very long time) until it reached the surface...
- ... where it formed into photons that started on their trip to the Earth
- Just to be partially absorbed and scattered by the atmosphere and water etc.
- With just a rather small part being absorbed by plants
Now the second part starts:
- The plants absorb a tiny part of the incident energy and store it in the form of sugars and fats. The efficiency of this process is something that's quite complicated to measure, but the typical number used is somewhere between 3-6% - that's absurdly horrible compared to anything human machines do.
- If you're lucky, when those plants die, they drop into a bog of some kind, where they can decompose slowly without oxygen.
- And then they are pushed deeper into the ground, increasing pressure and heat just to what's needed to form oil.
And the final, human part:
- The oil is mined (requiring more energy input as the well progressively depletes - most wells are abandoned long before they are drained simply because this becomes too expensive).
- ... and a small part of the original oil is refined to gasoline, the awesome fuel. We also find use for most of the rest, but looking at just the gasoline and diesel, you still throw away most of the refined oil which isn't the right composition for those.
The efficiency of the whole process is horrible. Gasoline is a great energy source today, but that's only because it's already here. Recreating it the way it formed in originally would be a huge waste of energy and time. We only retrieve something like 1-2% of the energy required in the first place. And once you have gasoline, it does actually decompose - if you don't purify it, it will no longer be good enough to power a car in a year or two. How does electricity compare?
A typical lithium-ion battery has an efficiency around 80-90% in reasonable conditions, along with a discharge of about 8% per month. A hydro-electric capacitor (basically two lakes joined by a pump that can either pump the water up using electricity or produce electricity as it goes back down) gets around 70-80%. This is where most of electricity is stored worldwide - reportedly around 99% (!) of spare electricity to deal with differences in day-night demand.
Gasoline is cheaper, but only because it was already made over millions of years in the past. We're reclaiming millions of years of work.
Safety is another tricky thing. Energy is dangerous - and efficient energy storage is also very good at discharging that energy by accident. Dams break. Gasoline burns. Batteries explode (and slowly self-discharge). Nuclear power seems almost trivial in comparison, if it were not for how concentrated (and thus "visible") their effects are on failure - using the typical technologies, one kilogram of fuel-uranium is used to produce as much energy as 1500 tonnes of coal. Of course, the uranium was first created in a supernova somewhere, using just tiny portion of the energy released in the explosion - if you wanted to use supernovae for "manufacturing" uranium, the efficiency would be even worse than coal :)
The thing is, neither gasoline nor diesel ignite easily, so it's relatively safe (and simple) to store them in large quantities. But while this makes them relatively safe, it also means that it's trickier to release the stored energy - compare an electric engine to a diesel engine. You can make electric engines as small as necessary (down to just a couple of individual atoms, it turns out!) - in fact, you could consider our own ATP-synthase ("the powerplant of the human cell") to be a tiny electric engine. The battery in your cell-phone likely has about one tenth the energy capacity of the same volume of gasoline - but it's vastly more practical for powering your cell-phone.
All in all, there are many competing characteristics of any energy storage:
- Charge/discharge efficiency - electrical storage is very good at this (usually using chemical potential as the "actual" storage mechanism)
- Long-term storage - great for non-rechargeable batteries, not so good for rechargeables - but still comparable to common gasoline.
- Environmental conditions - gasoline needs large machinery and high temperatures to be efficient, batteries work great at room temperature (and often lose efficiency when it's too cold or too hot) and are self-contained.
- Safety - gasoline doesn't burn easily and we have plenty of relatively safe ways to store it. Some batteries are safe, but the highest capacity/efficiency batteries are deathtraps. I hope you buy your LiPols from reputable manufacturers - mechanical damage, overcharging, overdischarging, temperature, improper voltage/current drain... there's plenty of ways to make them fail catastrophically, especially if their electronic safeguards are missing or not up-to-par :)
- Cost - gasoline is much cheaper than the same energy in a battery (though the comparison isn't simple at all - it's not very fair comparing an AA battery, considering the scaling issues; one gram of gasoline has about as much energy as a single AA battery). Rechargeable batteries are even more expensive, but quickly overtake gasoline over many charge/discharge cycles due to their high efficiency and relatively long life. And of course, gasoline becomes vastly more expensive if you consider making it "at home" from scratch (water + carbon dioxide), rather than from the fossil deposits.
Of course, the most direct storage of electric energy is a capacitor. They are incredible at both charge/discharge efficiency and peak power, but only deal with tiny amounts of energy density - the reason for that is that they fundamentally work with disbalanced charges, so there are pretty tight limits on how to hold those electrons that try very hard to push themselves apart (electromagnetic force is rather strong) together. There have also been some experiments with storing electricity directly in superconductive loops, though that has some obvious drawbacks :) As I said before, for bulk grid energy storage, pumped water is by far the most practical - at least for now. For consumer devices, chemical energy wins for the moment.